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Herd Immunity

(289 Posts)
Tabitha8 Sun 09-Sep-12 16:42:12

A simple title for what I think is probably a complex subject.

If we have herd immunity to an illness as a result of vaccinating our children, how is that maintained given that we don't vaccinate ourselves, the grandparents, our neighbours, etc?

JoTheHot Sun 09-Sep-12 17:19:11

Herd immunity is not an all or nothing effect. The greater the proportion of a population who is immune, by vaccination or other means, the greater the herd immunity effect will be.

Tabitha8 Sun 09-Sep-12 17:34:12

The idea of herd immunity is that it stops the spread of disease, isn't it? For example, stopping it spreading around the country.
So, unless a certain proportion of the whole population is immune (as you say, by vaccination or other means), the disease can still spread, can't it?

CatherinaJTV Sun 09-Sep-12 19:15:20

it depends on how closely people interact and how far they travel. School kids spread diseases best, but vacations often stop outbreaks.

JoTheHot Sun 09-Sep-12 19:23:17

The only thing which can stop a disease spreading is eradicating it. Herd immunity reduces the speed at which a disease spreads.

I found something the other day that suggested that Japan had lower vaccination rates for the chickenpox than the US but it was found that immunity from the vaccine lasted longer in Japan than in the USA. This was thought to be because the unvaccinated children gave the vaccinated children a boost to their immunity by providing contact with the chickenpox virus. The vaccinated children didn't get the illness but they did stay immune longer.

I think what that suggest to me is that by having some unvaccinated people within the community, we actually help boost the immunity of the rest of the community. Of course, that is pretty tough on the unvaccinated people because generally, the illnesses we are vaccinated against are not too bad in children but horrible for adults but short of vaccinating everybody, potentially having to give boosters and in the long term irradicating the disease, that would see the best way of maximising herd immunity. I think that we need those grandparents and neighbours to maximise the benefits of the immunisation programme.

I'll see if I can find the study that backs that up. It may be that it is only relevant to CP.

bumbleymummy Sun 09-Sep-12 22:05:13

The term 'herd immunity' can actually mean different things so it might be worth clarifying what everyone is actually talking about.

That's very interesting BBB. Would like to read your link when you find it.

ElaineBenes Mon 10-Sep-12 01:35:14

I think this explains herd immunity very well and why lower contact rates among older people means you can have lower immunity levels to achieve the same level of herd immunity.

herd immunity

JoTheHot Mon 10-Sep-12 07:57:56

bm, perhaps you could you give examples of different things 'herd immunity' can mean? All I can think is that perhaps you're confusing 'herd immunity', which only has one meaning, with the 'herd immunity threshold', a specific instance of herd immunity?

Elaine - I feel very dumb after reading that power point explaining herd immunity and possibly now know less than I did before I opened it. <Dances heads for the wikipedia>

bumbleymummy Mon 10-Sep-12 21:31:17

Here you go Jo :

"The term “herd immunity” is widely used but carries a variety of meanings [1–7]. Some authors use it to describe the proportion immune among individuals in a population. Others use it with reference to a particular threshold proportion of immune individuals that should lead to a decline in incidence of infection. Still others use it to refer to a pattern of immunity that should protect a population from invasion of a new infection."

Proposal to clarify the definition here

"The term herd immunity has been used by various authors to conform to different definitions. Earlier this situation had been identified but not corrected. We propose that it should have precise meaning for which purpose a new definition is offered: "the proportion of subjects with immunity in a given population". This definition dissociates herd immunity from the indirect protection observed in the unimmunised segment of a population in which a large proportion is immunised, for which the term 'herd effect' is proposed"

This uses it with the 'herd threshold' meaning:

"As measles is a highly infectious disease, the United Kingdom recommendation is for at least 95% of children to receive a first vaccination with the measles, mumps, and rubella (MMR) vaccine before age 2 years and a booster before age 5 years to achieve herd immunity and prevent outbreaks. "

ElaineBenes Mon 10-Sep-12 21:42:46

That's a very good article you linked to BM.

I particularly liked the paragraph on freeloaders

^“Freeloaders”
When vaccination has costs to the individual—side effects, time, money, inconvenience—individual decisions about whether to be vaccinated are based on a complex balancing of perceived costs of vaccination and disease. A high level of vaccine uptake in the community may mean that the chance of contracting an infection is close to 0. From the point of view of an individual, therefore, the ideal (selfish) strategy is that everyone else should be directly protected by vaccination, allowing the exceptional freeloaders to benefit from the indirect protection this provides.

Exploring this idea, vaccine choices can be considered using tools from mathematical game theory [30, 31], which show that when coverage is close to Vc, or when vaccination is perceived to carry a risk similar to or greater than the infection, the incentive for a logical individual to receive a vaccine is lowered [32]. One observes this in the declining measles and pertussis vaccine coverage in several countries with low disease incidence, after media scares about vaccines [33]. People are in effect performing complex cost-benefit analyses, based on imperfect assumptions (for example a failure to appreciate the complex relationship between age and clinical severity of infections), when deciding whether or not to have themselves or their children vaccinated. It is not surprising that a sustained low incidence of infection, caused in large part by successful vaccination programs, makes the maintenance of high vaccination levels difficult, especially in the face of questioning or negative media attention.^

bumbleymummy Mon 10-Sep-12 21:51:35

Yes, I think cote d'azur has previously posted about Game Theory in relation to vaccines.

ElaineBenes Mon 10-Sep-12 22:06:58

Right - but as the author says, people are making imperfect assumptions based on media attention without fully appreciating the risks involved and also the vaccination programmes are victims of their own success. I think they're spot on.

Tabitha8 Mon 10-Sep-12 22:09:30

Oh dear, oh dear, more complex than I had assumed!

People like me, who haven't vaccinated their child/children don't choose that course of action on the basis that everyone else has been vaccinated. However, I don't think the article was suggesting that in the "freeloaders" paragraph.

This, also from Bumbley's link, is interesting as well:
Similarly, waning vaccine-induced immunity demands higher levels of coverage or regular booster vaccination. Important among illustrations of this principle are the shifts to multiple doses (up to 20) and to monovalent vaccines in the effort to eliminate polio in India, where the standard trivalent oral polio vaccines and regimens produce low levels of protection.

bumbleymummy Mon 10-Sep-12 22:16:53

Yes Tabitha. I also read another article on herd immunity yesterday (WHO or HPA - must check history on computer when I'm next on it) which recognised that waning immunity from the vaccine is an issue. The current whooping cough epidemic is proof of that. So, as you said at the start, if older children/teenagers/adults aren't having boosters and their immunity is waning then ...

ElaineBenes Mon 10-Sep-12 22:17:33

tabitha

actually that's exactly what the article is suggesting - just in a non-judgmental way

how on earth can you make a rational cost-benefit analysis of whether to vaccinate or not if you haven't taken in to account the actions of those around you!?

ElaineBenes Mon 10-Sep-12 22:20:03

Waning immunity IS indeed an issue for some vaccines.

And that is why we need more boosters and/or better vaccines - as well as higher coverage to achieve herd immunity. Teenagers should have pertussis boosters. Infants should be protected by ensuring all around them are immune to pertussis. Not just not vaccinating!

It is not a reason to raise one's hands and say 'oh well, guess we'd better ditch the whole vaccination programme!'

Tabitha8 Mon 10-Sep-12 22:23:16

But Elaine I can't base my decision on the actions of others when it comes to vaccination as I don't know what what they have all done. I haven't done a survey around where I live.

bumbleymummy Mon 10-Sep-12 22:23:22

Actually, they are thinking of vaccinating babies against pertussis to offer them earlier protection. It makes more sense than trying to vaccinate everyone around them.

Some studies have shown that pertussis immunity from the vaccine wanes in as little as a year EB and I think the average is around 4-6 so giving teenagers a booster would still leave a bit of a gap.

seeker Mon 10-Sep-12 22:24:40

"People like me, who haven't vaccinated their child/children don't choose that course of action on the basis that everyone else has been vaccinated"

Not consciously, maybe. But your decision is based, among other things, on the fact that you have never seen, for example, diptheria or polio in a child because both have been eradicated in this country because of vaccination.

Tabitha8 Mon 10-Sep-12 22:27:40

Yet there are so many people in the country who have never had a polio jab so how did that contribute to eradication?
Re whooping cough - isn't it primarily a big health concern for young babies, hence the idea of vaccinating them earlier? It wouldn't be a particular problem for an older child or adult, would it? Generally speaking, you understand?

ElaineBenes Mon 10-Sep-12 22:31:00

BM

Might leave a gap but as you've seen in the mathematical models, the more who are immune, the less the disease can circulate. It might not be perfect but it's a lot better than doing nothing.

And I agree it'd be great if the pertussis vaccine could be given at an earlier age as that is when protection is most needed. But in the absence of such a vaccine, a cocoon of protection is the next best option to protect young babies from whooping cough.

Tabitha

It's not hard to get data on vaccination rates. And, as seeker pointed out, you know that your children are unlikely to get diptheria because others DO vaccinate against it. Also, it's very important to know whether not vaccinating your child - for example against measles or mumps - won't push the average age of infection for non-vaccinated children up. So if MMR rates drop enough to allow the diease to spread but not sufficient to be sure your dc will contract the disease pre-puberty, what then? Seems to me you've made your decision without full information and put your dc at risk.

seeker Mon 10-Sep-12 22:31:52

"Yet there are so many people in the country who have never had a polio jab so how did that contribute to eradication?"

I don't understand. Most people born since 1960ish have been vaccinated. And as it is mostly a disease of children, how can you say that vaccination hqsn't been th cause of irrqdicqtion?

ElaineBenes Mon 10-Sep-12 22:32:25

Tabitha - why do YOU think the UK is a low risk country (not eradicated though) for polio whereas it was a public health problem in the 1950s? Why do YOU think polio has been eradicated from the US and Canada?

bumbleymummy Mon 10-Sep-12 22:35:37

At the moment we still have a significant proportion of the adult population who are naturally immune to things like mumps and measles. I wonder what will happen as the vaccinated with their waning immunity start to make up a larger proportion of the adult population.

bumbleymummy Mon 10-Sep-12 22:37:29

Ali, it's completely unrealistic to think you can 'cocoon' a baby by vaccinating an entire population with a vaccine that only provides limited protection.

bumbleymummy Mon 10-Sep-12 22:37:46

Also* not Ali

ElaineBenes Mon 10-Sep-12 22:40:44

I'm sure if it becomes an issue, it'll be possible to introduce a booster. But I haven't heard of measles outbreaks in vaccinated adults

ElaineBenes Mon 10-Sep-12 22:41:33

No, you vaccinate those the baby will have close contact with. Not perfect but - again - lowers the risk significantly and better than doing nothing and allowing babies to die of pertussis.

AnitaBlake Mon 10-Sep-12 22:42:07

Tabitha, you are right, the main group in danger is very young babies, there have been nine babies died of whooping cough this year in this country. Waning immunity plays a part, choosing not to vaccinate a further part.

I'm not entirely sure how diseases are eradicated but vaccination drives them out somehow, given the massive widespread global lack of diseases like polio and smallpox, it certainly seems to work.

bumbleymummy Mon 10-Sep-12 22:47:31

Considering the high rate of vaccination for pertussis I think we can say that waning immunity plays a much bigger part Anita.

bumbleymummy Mon 10-Sep-12 22:49:20

So vaccinate all siblings (again), parents, grandparents, aunts, uncles, cousins (again) etc. The vaccine companies must love you EB.

ElaineBenes Mon 10-Sep-12 22:50:08

Not vaccinating is the greatest risk to any one individual child.

seeker Mon 10-Sep-12 22:52:27

Anyway. Back to polio. It was a constant terror to my mother and her friends when I was a child. Unheard of in this country now. And vaccination was introduced in the 1960s.

Or iw that just a coincidence?

I didn't think polio was administered by a jab Tabitha - it is administered by drops which is perhaps why you don't think it is still being given. It is. In the old days on a sugar cube. I would say that most people have had the vaccination. I even had a booster as an adult although I can't now remember when, perhaps signing on with a new doc in my early 20's perhaps. I know I wasn't expecting it. Same time as the tetanus booster. My DC have had it too.

Anyway, polio has nearly been irradicated by a global vaccination programme which I suspect (I haven't checked) very few other diseases have had the benefit of. It is a very serious disease compared to something like mumps or rubella though. I think the focus was entirely justified.

ElaineBenes Tue 11-Sep-12 01:32:48

Funnily enough one of the reasons polio hasn't been eradicated in west Africa is the misinformation and anti-vax campaign in northern Nigeria. Anecdotal claims of vaccine damage, conspiracy theories, media focus - sounds frighteningly similar to the mmr scandal in the uk!

sashh Tue 11-Sep-12 02:21:57

Tabitha

If you were moving to Africa would you have your children vaccinated?

And most adults had live polio vacine in the form of drops although it can be given by a jab.

seeker Tue 11-Sep-12 06:53:24

I was using "vaccination" to include the orally administered polio vaccine. "immunisation" might have been a beet word.

LeBFG Tue 11-Sep-12 12:42:17

Stop painting a one-sided view bm. Naturally-aquired whooping cough immunity wears off too. A study gives it from as little as 4 years. But you know that - I've reminded you on other threads. And I have to second Elaine's point - so what if a vaccine is imperfect? That is no good argument to not vaccinate (but then we've been there too ad nauseam). There is a tiring slog of one-sided posts from you on this board, always arguing the counter-point. With what motivation?

Game theory has indeed been brought up before. It is used to model behaviour based on perceived risk, not actual risk. So, Tabitha, you may feel like you have done your research and reached an independant conclusion, but in fact every bm you bump into convinces you of a risk that may either not exist or is over-hyped. Your choice isn't independant of others (so little of what we choose is tbh). And the more people in RL or on here that do the same as you just confirms your choice. If you don't like this idea, you might be happy to know this would also work in reverse. In a situation where the DM and it's ilk went crazy over a sustained period about the terrible consequences of contracting, let's say, hand, foot and mouth, vaccinations (if they existed) would sky rocket as people would be over-estimating the risk of contracting this relatively innocuous disease.

Tabitha8 Tue 11-Sep-12 22:14:47

Goodness, where to begin.
Polio.
BBB Yes, I know a polio jab is still given to children. As part of a 5 in 1 or similar.
Elaine by mass vaccinating against mumps, are we not pushing up the average age when people contract it? Because the MMR mumps part wears off the soonest? That's why we've seen outbreaks of mumps in universities, isn't it?
Seeker I know many, many people born before the introduction of the polio vaccination. They may have had polio as children. My parents only remember having measles and scarlet fever. I've only met one person who suffered permanent damage as a result of catching polio as a child.
Elaine You said it's easy to get vaccination data. Yes, so? I do not know the vaccination status of the children that my child plays with. We don't discuss it. I don't know the mothers well enough. Friends of mine with chidren have either vaccinated fully their children (they live too far away for that to matter to me) or their children have long since grown up.
Elaine you ask me why I think the WHO has been able to declare Europe polio free. I don't know. You want the vaccine to take the credit. Fair enough if you do. If I were to quote from the "crankosphere" you'd jump on me, so I won't.
BBB You describe polio as being very serious, yet I've only met one person who suffered permanent damage from it, out of all the pensioners I know and have met in the past.
Sassh If I were moving to Africa? Goodness knows. It would depend on to where in Africa. If to a part with raging poverty, unclean water, and the consequent poor health.......... How about holiday vaccinations? That's a more realistic scenario. I'd have to get back to you.

Tabitha8 Tue 11-Sep-12 22:17:51

LeBFG I assure you that no one convinced me not to vaccinate my child. I'm not completely convinced about my decision myself, and have been quite open about that before. I see the whole area as a risk whatever decision that I make.

Now, will probably have to post and run as usual smile.

Goodnight all.

seeker Tue 11-Sep-12 22:49:02

Before polio was eradicated in this country, this was how it worked. Some people got it. Some of those died. Some were paralysed and some recovered. The ones who recovered then had natural immunity. Of the rest of the population, some never came into contact with it, and the rest did, but acquired natural immunity by having the disease so mildly that it as probably not diagnosed. The pensioners you meet today fall into the natural/acquired immunity or never came into contact groups.

Just because you don't personally know anyone who has suffered long term effects does not mean that it wasn't- and isn't in some countries- a serious illness. Ask anyone who had children before 1962 about it. Oh, and about diptheria too.

LeBFG Wed 12-Sep-12 06:44:15

I'm also pretty sure that not one person convinced yo, or persuaded you, or influenced your descision...it was a catalogue of encounters, things read on website, relations or friends with stories etc the sum of which influenced you to make the descisions you did. As I said, this is true of lots of what we decide, including deciding whether it's leggings or flares that look cool this winter.

seeker Wed 12-Sep-12 08:43:54

And one of the things that will have tipped you in the non vaccination direction is the undeniable fact that the scary illnesses that terrified my parents (who were the same age as most mumsnetters grandparents) are just not around any more. If you had actually seen a child with diphtheria, or polio or tuberculosis or a really bad case of measles, or even had, as I had a first hand account of one of those, you might well have been tipped in a different direction.

LeBFG Wed 12-Sep-12 08:55:18

Seeker, when these debates come up, I'm always reminded of people who live in flood plains or overshadowed by a live volcano. They repopulate these areas the world over, quickly forgetting in a few decades the dangers of living so close to such dangers. Then, boom, one day the disaster happens. In some respects we are a forgetful species.

seeker Wed 12-Sep-12 09:17:34

Maybe part of ante natal care should be a tour of a pre vaccination cemetery? All those heartbreaking lists of names........

Do we not have to forget sometimes? The world would be a very scary and unhappy place if we lived in the shadow of all these fears, wouldn't it? I've vaccinated my children to the usual childhood illnesses because they are so awful but now they are not a direct threat to me, I don't have to remember how bad they were - even though I am not daft enough to be uneducated about it on the very rare occasions that I am forced to think about them.

I suppose that herd immunity comes from us all sticking together and doing the same thing (vaccinating mostly) so that we don't have to think about the risks. We just do it because the rest of the herd does it. For herd immunity to work that is the best thing.

Tabitha - serious illness to me means something that is serious whilst you are suffering it or which takes a long time to recover from, it doesn't nessarily mean that it will do permanent damage even though it can in a fair proportion of cases. To me chickenpox is no big deal for the majority of people but it can have serious consequences in some. Polio is always a serious illness when you have it and it is much more likely to leave permanent damage. Not all illnesses are equal. The fact that you might not suffer permanent damage is only part of it. The other thing to ask is if you would want to catch the disease in the first place.

And you know don't you, that just because you do or don't personally know somebody who has suffered, doesn't really make a very good statistical sample. I don't think I know anybody personally who has suffered polio. Doesn't mean that I don't think there is no point vaccinating against it. In fact it is because I don't know anybody who has suffered that I know the vaccine is a good idea - clearly it works! Some vaccines are not quite so effective but that one is so why not have it?

MordionAgenos Wed 12-Sep-12 11:13:42

@seeker Polio vaccination was introduced in the 1950s. My DH contracted polio the week before he was due to be vaccinated (at age 3 or 4) in 1957, in the last big London epidemic. He was very lucky to emerge relatively unscathed.

My mum had diptheria as a small child. I've never forgotten her telling me and my sister how she would never forget hearing the doctor declaring her dead (obviously she wasn't dead, he was just overworked and probably on the point of collapse himself coping with a diptheria outbreak in a scuzzy bit of inner London)

And DS missed practically the whole of last term at school due to whooping cough (and he has had ALL his vaccinations, like all my kids).

We are BIG believers in vaccinations in my family.

MordionAgenos Wed 12-Sep-12 11:27:23

@Tabitha believe me if you had met anyone who had polio as a child then they would know about it (and so would you). But there's a reason why you don't meet many of them. Most of them died and many of those that didn't die while they had it died not long afterwards. The prognosis for life in an iron lung (which was where many of the survivors ended up) wasn't long.

sashh Wed 12-Sep-12 11:41:36

Tabitha

That's intersting. OK try 'would you have (let your children have) holiday vaccinations.

bumbleymummy Wed 12-Sep-12 12:17:32

LeBFG, as I've previously pointed out, no one is demanding that we all expose ourselves to whooping cough to gain natural immunity and protect the 'herd'. If you accept that natural immunity can also wane (although in general it lasts longer, as you are well aware) then I'm not really sure how you can put forward a convincing argument for creating herd immunity with the vaccine knowing that it is even less effective.

BBB, polio isn't always serious. 95% of people will have no or mild, flu like symptoms and may not realise they have polio. Around 1% may have paralytic polio but the effects aren't always permanent. Obviously the paralytic form can be very serious and I'm not belittling that. I just wanted to point out that isn't 'always a serious illness'.

I realise it is asymptomatic is 90-95% of people but then there are a lot of illnesses that we carry that we don't know about so I wouldn't really count them as having had the disease even though I suppose medically we have. It is still serious in those who get it - I don't call 'flu-like symptoms particularly mild. Flu makes you feel like crap so I am told - I don't know for sure as I am fortunate never to have had it. I have had some terrible colds but never flu.

Of the people who get symptoms a good proportion of those do get the more serious sypmtoms too - apparently 20-40%. That is a lot when you compare it to the percentage that get complications in other diseases like chickenpox.

seeker Wed 12-Sep-12 13:06:24

Bumblymummy-this is a serious question. Would you be happy for your child to go unvaccinated into an area where there were active polio cases?

bumbleymummy Wed 12-Sep-12 13:26:22

BBB, it is possible to have mild flu. The mild flu like symptoms they mention are headache, tiredness etc. I'm by no means trying to say it is in the same league as CP but saying 'it is always serious' or 'most of them died' isn't quite accurate.

Well, they wouldn't have much reason to go to those areas and tbh if they did I would be more worried about things other than polio!

seeker Wed 12-Sep-12 13:32:44

Really? So your child, for example, decides to do DfoE or something similar in areas where polio is endemic and you wouldn't worry about it?

LeBFG Wed 12-Sep-12 13:55:35

The vaccine maybe less effective bm but there are at least three things you are overlooking.

One, herd immunity increases with increased number of immune people. Wrt wc, herd immunity effect is clearly in operation as "Mortality and morbidity have fallen drastically in parts of the world where toddlers have been systematically protected with efficacious whole-cell vaccines"

Two, when people are vaccinated, they are not required to become ill in order to create herd immunity.

Three, both naturally-aquired and vaccination immunity durations are highly variable, in some people lasting as little as 4-7 years and in others up to 12-20 years.

These factors (and others, such as severity after re-infection with wc) mean the case is not as simple as you make out bm.

BM - We are quibbling over what serious means. Mild flu isn't really flu in my book. It wouldn't register as such anyway. Nowhere have I read that those with symptoms that are like 'mild flu', just plain old fashioned flu, headache temperature, sore throat, gastric problems etc, and therefore not very nice.

I'll assume that you aren't attributing the 'most of them died' comment to me as well since I haven't said that. It isn't true!

Your children may make the decision to go to the places where polio is endemic long after you have control over where they go. Will you tell them why they are unprotected. Perhaps they will join the armed forces and have to go to places that people from this part of the world probably wouldn't venture. Or just hope that because the vast majority of us have had our children vaccinated yours don't need to bother because the disease has been wiped out.

LeBFG Wed 12-Sep-12 13:57:27

No point asking questions like that to bm, seeker. Bm would be happy to live in modern world without vaccines. Isn't that right bm.

bumbleymummy Wed 12-Sep-12 14:52:20

Are you saying that we have herd immunity to whooping cough at the moment LeBFG? Perhaps you should clarify what you mean by herd immunity (see above). I'm not sure why you have used your quote as an illustration of herd immunity. Wouldn't you just attribute that to the vaccine offering some protection to the child itself? I'm not overlooking those things btw. I'm not sure why you think I am.

Seeker(and BBB) at that age it would be up to them anyway but, as I've already said, I would have bigger worries!

BBB, I'm not saying they're nice but obviously those symptoms can be mild or more severe. There is a scale. No, I wasn't attributing the 'most of them died' comment to you - just pointing it out as incorrect.

BFG, I see you're still twisting things said on a previous thread. Let's just say I probably wouldn't be panicking about it as much as some of you would smile

seeker Wed 12-Sep-12 14:59:42

"Seeker(and BBB) at that age it would be up to them anyway but, as I've already said, I would have bigger worries!"

I do find it fascinating that people who are opposed to vaccination will never answer a straight question like this. I've asked it frequently, both on here and in real life, and I have never had an answer.

bumbleymummy Wed 12-Sep-12 15:01:40

I've answered you twice now confused. Maybe you just aren't getting the answer you want.

seeker Wed 12-Sep-12 15:09:14

No you haven't. You've said it would be their choice and you would have other things to worry about.

You haven't said whether you would be happy for your unvaccinated child to go into an area where polio was endemic- as it was in this country until the early 60s.

LeBFG Wed 12-Sep-12 15:15:14

Herd immunity, as it is correctly used and widely understood in the literature is NOT herd immunity threshold. I would highly recommend you read the wiki definition if you have problems understanding this.

Some sort of herd immunity to wc exists because babies too young to be vaccinated are dying less and are ill less than in the pre-vaccination era. This is indisputable (except by Prof bm perhaps). That there is now a growing incidence of wc more recently is another issue. Wc herd immunity exists despite the fact the vaccine in less than perfect.

bumbleymummy Wed 12-Sep-12 16:13:20

I posted quotes and links earlier about the various definitions of herd immunity including a link to a study that used it in the context of herd immunity threshold. Perhaps you should refer them all to wiki?

In any case, most literature will tell you that a significant proportion of the population need to be immune for there to be any herd effect. Why do you think the growing incidence of WC is a separate issue?

bumbleymummy Wed 12-Sep-12 16:18:49

You might have to refer these people to wiki too.

seeker Wed 12-Sep-12 16:23:08

Why are we debating the meaning if 'herd immunity"? Surely we all know what it means for the purposes of general discussion?

Still no answer, bumblymummy?

bumbleymummy Wed 12-Sep-12 16:38:08

Doesn't it make sense to clarify what we are talking about for the purposes of the discussion?

I've already answered you seeker. I wouldn't be happy for them to go there but it wouldn't be polio that would be worrying me. If there was an outbreak of polio here, in the UK, would I be worried - no.

MordionAgenos Wed 12-Sep-12 16:52:59

@bumbly The 'most of them died' comment was made by me and was correct but I didn't make it clear I was referring to the respiratory type of polio (which is the type my DH had). That's the type that put people in iron lungs. That is the really scary type (weird to think that the type of polio that could result in being in callipers all your life, as happened to two by then adults I knew when I was a kid, was actually the 'good' type of polio).

The number of people who have a memory of the last great UK epidemic is diminishing all the time. The number of people who had first hand involvement is even smaller. The number of people with relatives who were involved is obviously greater but still - it's very unusual, especially in certain parts of the country (less so in London, or some of the other worst hit centres in the late 50s I suppose). DH is the only person our family GP (who is older than me but who doesn't come from London) has ever known who had polio.

MordionAgenos Wed 12-Sep-12 16:53:58

The reason I'm going on about this is because some posters are being ridiculously ludicrously dismissive of polio and its possible impact.

LeBFG Wed 12-Sep-12 16:56:16

Your last link in no way contradicts what I just said: more people immune, greater the herd immunity. Pretty obvious really. To ask your question back: do you think we have herd immunity to wc?

You may (or may not) be interested to know that there is research into the effect of lower transmission rates leading to more virulent wc presentations. This is not to do with herd immunity but simply that more people are immune now due to vaccinations so transmission of the disease has lowered.

LeBFG Wed 12-Sep-12 17:03:46

Oh, some paper may define a parameter x and call it herd immunity somewhat inaccurately btw for ease of reading. Or refer to the threshold effect as simply herd immunity. That was why I said 'as it is correctly used and widely understood". The wiki definition is highly uncontroversial. For ease of understanding, the wiki definition should be used imo when talking about herd immunity. The herd immunity threshold has a very specific meaning and is another thing.

seeker Wed 12-Sep-12 17:27:28

"I've already answered you seeker. I wouldn't be happy for them to go there but it wouldn't be polio that would be worrying me. If there was an outbreak of polio here, in the UK, would I be worried - no."

Could you explain why? Assuming the outbreak was local to you?

AnitaBlake Wed 12-Sep-12 18:03:32

Sorry, just got back to this. No-one is quite sure why the wc epidemic is happening now. I wonder if it had anything to do with the wc vacc scare in the 70's and 80's when vacc rates fell to 30%. Those babies are parents now. Even last year most cases of wc were in unvacc'd kids (at least the ones I heard of at work were).

I must also apologise about the wc death rate I quoted. After speaking with the vacc specialist at work today, nine babies have died this year in this country due to wc. There are intentions to start vaccing pg women to try help reduce the incidence in babies.

ElaineBenes Wed 12-Sep-12 18:21:30

Anita - In the US, the recommendation is to create a 'cocoon' around a newborn by vaccinating/giving a booster against wc to parents and siblings and others who will be in extended close contact with the baby. They also recommend giving a booster to young teenagers as routine.

Probably the increase in wc cases is a function of both vaccine induced immunity declining faster than expected and the proportion of the population unvaccinated. The two together create a bit of an unholy alliance.

AnitaBlake Wed 12-Sep-12 18:31:37

Agreed EB, I'm not an expert, but I am getting facts direct lol. I think the cocoon idea is an excellent one, and most likely being looked at by those higher up. There must be more to the current outbreak than simply waning herd immunity given the vacc has been given for many years, this should have presented earlier than now. Even the experts agreed they aren't sure why its happened.

The rise in measles is much more easily explained.

bumbleymummy Wed 12-Sep-12 19:18:02

This part (from my last link) contradicts what you are saying:

"This critical percentage varies according to the disease, the interactions between members of the population and the vaccine, but 90 per cent is not uncommon. This is herd immunity - the fact that others in the herd or population are vaccinated provides protection to all, whether or not vaccinated themselves."

So does this

"Herd immunity

Herd immunity is the proportion of the population that is immune to a particular infectious disease.

Herd immunity threshold (HIT)

The HIT is the proportion of a population that need to be immune in order for an infectious disease to become stable. If this is reached, for example, due to immunisation, then each case leads to a single new case and the infection will become stable within the population. That is, (R=1).

In addition, if the threshold is surpassed then R<1 and the disease will die out."

We currently have a whooping cough epidemic so I'm thinking that R > 1 at the moment.

Morrison, I am not trying to belittle your husband's experience or anyone else's for that matter. I said in a PP that it can be very serious but I'm sorry, you are incorrect in saying that 'most of them died' even in relation to paralytic polio which makes up around 1% of cases.

Here is a link for you

"
In about 95% of all polio cases, the person has no symptoms at all. These are known as asymptomatic cases. The rest of polio cases can be divided into three types: abortive polio, non-paralytic polio, and paralytic polio.

Abortive polio: In these cases, polio is a mild illness, with viral-like symptoms such as fever, fatigue, headache, sore throat, nausea, and diarrhea.

Non-paralytic polio: These cases typically involve the symptoms of abortive polio, with additional neurological symptorms, such as sensitivity to light and neck stiffness.

Paralytic polio: The first signs of paralytic polio, after an initial period of viral-like symptoms, typically begin with loss of superficial reflexes and muscle pain or spasms. Paralysis, usually asymmetric, follows. Fewer than 1%-2% of people who contract polio become paralyzed. In most cases of paralytic polio, the patient recovers completely. However, for a certain number of people, paralysis or muscle weakness remains for life.

...

Complications and Mortality Rate

In severe cases of paralytic polio, the throat and chest may be paralyzed. Death may result if the patient does not receive artificial breathing support. Between 2%-5% of children affected with paralytic polio die, whereas for adults, 15%-30% die. "

Tabitha8 Wed 12-Sep-12 19:24:32

re polio again.
If it broke out here, it wouldn't be a huge worry for me, I have to admit (ought that to be hate to admit?)
I don't know my immunity status to it. If I caught it and had even very bad flu like symptoms, I'd cope.
As I said before, I did know someone who had permanent problems from polio – she had a limp. Wasn’t there a singer who had it similarly as well?
As Bumbley has pointed out, it's unlikely that I would be left permanently paralysed.

If my child were to join the army as BBB mentioned I’m sure he’d take the advice of army doctors with regard to vaccinations. However, I would expect the army to keep him well nourished and to give him clean water to drink. I’m not sure myself why vaccines ought to be needed in such circumstances, but then my reading over the past three years has only focussed on this country.

Which brings me back to holiday vaccinations. Hmm. As I said, I’d have to look at the country and the list of suggested vaccinations. I know nothing, for example, of cholera. I can’t even spell it. I know that it was dealt with here by providing clean water. I suspect I’d simply not go on the holiday. I’ve had a cholera jab and it was horrendous in terms of what it did to my arm for a day or two. If I were in that situation again, I’d ask myself what the chances were of a tourist ending up with cholera.

MordionAgenos Wed 12-Sep-12 19:49:09

I note you didn't quote the mortality rate for respiratory polio. The full quote reads:

----

Without respiratory support, consequences of poliomyelitis with respiratory involvement include suffocation or pneumonia from aspiration of secretions.[56] Overall, 5–10% of patients with paralytic polio die due to the paralysis of muscles used for breathing. The mortality rate varies by age: 2–5% of children and up to 15–30% of adults die.[4] Bulbar polio often causes death if respiratory support is not provided;[39] with support, its mortality rate ranges from 25 to 75%, depending on the age of the patient.[4][60]
----
I was talking about bulbar, aka respiratory, polio as I made clear upthread. The sort of polio that landed kids in iron lungs.

But hey - you clearly don't believe in it. Go you.

bumbleymummy Wed 12-Sep-12 20:13:03

Saying 'respiratory', is misleading because spinal polio can also effect the lungs. Bulbar polio accounts for 2% of paralytic polio cases so if you want to say, '25-75% of the 0.02% of people who contracted bulbar polio died' then go ahead.

ElaineBenes Wed 12-Sep-12 20:14:32

BM

The threshold is critical for determining whether a disease will continue to spread in a population or whether it will die out. If 1 person, on average, spreads the disease to less than 1 person, any outbreak will be localized and not sustained. If it is just below 1, the transmission chain will probably be longer than if it is way below 1.

Similarly, even if 1 person spreads to the disease to more than 1 person, herd immunity will affect the rate of spread and the 'peak' of an epidemic. If it is just above 1 the rate of spread will be much slower than if significantly above 1.

You don't even need to 'believe' in vaccinations to understand this. Why, historically, has the average age of infection been high in rural populations compared with urban ones?

Why is this so hard to understand BM?

LeBFG Wed 12-Sep-12 20:27:27

<apologies in advance for the essay>

BM: I read the bit in the first link: "Any vaccine will be more effective at the population level if more people have been vaccinated..." and agreed to that.

"This critical percentage varies according to the disease, the interactions between members of the population and the vaccine, but 90 per cent is not uncommon. This is herd immunity - the fact that others in the herd or population are vaccinated provides protection to all, whether or not vaccinated themselves."

This does in fact describe herd immunity threshold and yes, someone should be refering them to wiki.

For the second quote:
"Herd immunity is the proportion of the population that is immune to a particular infectious disease." Agree completely. When the proportion of the population who are immune is zero, there is zero herd immunity, thus zero protective effect. Simple.

Where did all this begin? As is usual with bm, had to scroll upthread and find the nugget at the heart of the dispute. Bm, you said to me: "I'm not really sure how you can put forward a convincing argument for creating herd immunity with the vaccine knowing that it is even less effective." Despite it being less effective, there is still an effect! How do I know? Because wc was the leading cause of death in babies the 1930's in the UK, just prior to the vaccination campaign! Every country that has introduced a vaccination campaign has seen wc deaths in babies plummet!

Unfortunately, the word 'epidemic' is slippery. It varies depending on the disease being described. Do you have a link that showing the UK is currently experiencing a bona fide epidemic of wc? I would be interested to know. I thought there were localised outbreaks, showing the herd immunity effect to be going strong in many parts of the UK. As the person upthread said, the outbreaks of wc are puzzling as we've had low rates for so long (clearly herd immunity has been doing it's bit for a long time, despite the fact the vaccine doesn't confer life-long immunity). There are clearly multiple factors at work.

bumbleymummy Wed 12-Sep-12 21:25:49

Elaine, in order for a disease to become stablein the community, R must be <= 1. If it is greater than 1 then the disease will continue to spead until it again reaches a point where R=1 due to people gaining natural immunity and no longer being susceptible to infection. The thing is, just because someone has been vaccinated, it does not mean they are not susceptible. In the case of WC, older children and adults who were vaccinated when they were younger are no longer immune and are therefore susceptible to the disease, allowing it to spead.

BFG, I think you're confused by the meaning of second quote. I'll refer you back to what I quoted earlier:

"The term herd immunity has been used by various authors to conform to different definitions. Earlier this situation had been identified but not corrected. We propose that it should have precise meaning for which purpose a new definition is offered: ^ "the proportion of subjects with immunity in a given population". This definition dissociates herd immunity from the indirect protection observed in the unimmunised segment of a population in which a large proportion is immunised, for which the term 'herd effect' is proposed^"

That second quote supports the above proposed definition and therefore dissociates it from the protective effect offered to the unimmunised. So what are you actually saying you think it means?

My comment earlier, taken in context, is that you seem to think that herd immunity to whooping cough would not be possible with the natural infection due to waning immunity yet you think it is possible with the vaccine which, on average, does not last as long as natural immunity.

Wrt whooping cough in the UK, most official sources are saying 'outbreak'. The media seem to be the ones saying 'epidemic'.

seeker Wed 12-Sep-12 21:32:56

Sorry- are people saying that they wouldn't be worried if their child was exposed to a disease that could leave them dead, or with varying levels of paralysis? Really?

bumbleymummy Wed 12-Sep-12 21:39:16

Seeker, the chances of any of us being hit by a car and being killed/left paralysed are far far greater yet we don't worry about that every day.

ElaineBenes Wed 12-Sep-12 21:46:47

Ah, so this is the crux of the matter BM. A person who has been vaccinated is far far far more likely to be immune and therefore not susceptible.

No, it's not 100% but this is all about probabilities. You seem to think it's all or nothing.

Vaccinated people are, on average, less susceptible to a disease than unvaccinated people who have not been exposed to the disease. Agree? You may argue the degree but the concept is solid.

Therefore by vaccinating you reduce the susceptible population. The density of susceptibles is reduced and you reduce the chance of an epidemic starting and, if it does, reduce the severity of the epidemic. Again, you may argue the degree (although it would seem to work with, say, measles) but the maths is solid. Right?

So why do you claim that vaccine induced herd immunity doesn't exist?

ElaineBenes Wed 12-Sep-12 21:47:20

Actually, I do worry about it BM. That's why I strap my kids into their car seats.

seeker Wed 12-Sep-12 21:48:01

Hang on, my head's spinning a bit here. Are you saying that vaccination is unnecessary because any one individual is unlikely contract a serious illness in their lifetime? The reason you don't think people should be immunised against polio is because not many people were seriously affected by it? This is a new sort of thinking I haven't come across before.

ElaineBenes Wed 12-Sep-12 21:51:39

I sometimes I feel like I've discovered a parallel universe, seeker

bruffin Wed 12-Sep-12 21:58:53

It wasn't not many people who were affected by it, it was 1000s a year in UK and 10s of 1000s on the US. Not insignificant numbers.
The risk from vaccinating is far less than the risk of the disease, unless of course you are relying on every one else vaccinating.

seeker Wed 12-Sep-12 21:59:26

I get really incandescent with rage sometimes when I think of mothers in the developing world carrying their children for bloody miles in the hope of vaccinations which might actually give them a chance of reaching adulthood, while us comfortable Westerners burble on about polio being nothing much to worry about.

bumbleymummy Wed 12-Sep-12 22:01:39

EB, even your own link talks about reducing the number of susceptibles below a level in order to achieve herd immunity. You seem to talk about herd immunity as if it exists regardless of the number of susceptibles. Is that what you think? That as soon as you have a few immune people in a population then you have herd immunity?

bumbleymummy Wed 12-Sep-12 22:08:27

EB, You know you can be hit by a car when you aren't in a car don't you?

Seeker, I get incandescent with rage when I think about mothers having to walk miles to get clean water for their children to drink when we can just turn on our tap. Clean water would save a hell of a lot more lives.

bumbleymummy Wed 12-Sep-12 22:09:09

Bruffin, what are you replying to?

seeker Wed 12-Sep-12 22:09:52

"Is that what you think? That as soon as you have a few immune people in a population then you have herd immunity?"

Nobody thinks that.

Do you think that you children are somehow magically protected from the killer diseases that used to fill our church yards?

seeker Wed 12-Sep-12 22:11:42

"Seeker, I get incandescent with rage when I think about mothers having to walk miles to get clean water for their children to drink when we can just turn on our tap. Clean water would save a hell of a lot more lives."

It's not either/or.

I'm pretty sure Theodore Rooseveldt and Michael Flanders had access to clean water. Didn't stop them ending up wheelchair users though.

bumbleymummy Wed 12-Sep-12 22:13:38

I'm glad to hear it seeker because plenty of people seem to keep saying 'it's not all or nothing' as if the threshold doesn't even come into it.

'magically protected' no. Less likely to contract and/or be seriously damaged by them due to better nutrition, sanitation and healthcare, yes.

seeker Wed 12-Sep-12 22:15:08

Ian Drury had access to clean water too.

bumbleymummy Wed 12-Sep-12 22:18:02

I'm talking about clean water in developing countries as you are well aware. Don't try to belittle their situation. Children are dying every minute because they don't have access to clean water - you can't vaccinate against that.

bruffin Wed 12-Sep-12 22:20:25

You are belittling every child that has died or been affected by a vaccine preventable disease with every single post you write.

seeker Wed 12-Sep-12 22:20:39

I wasn't trying to belittle anything. As well you know.

ElaineBenes Wed 12-Sep-12 22:21:55

Umm, yes, BM. Even a few people vaccinated would reduce the number of susceptibles - by a very little and probably not enough to really notice when outweighed by huge numbers of susceptibles.

I don't know enough to know if the relationship is linear, I suspect it's more an S shaped relationship but I'm not sure.

Why is this is an issue for you? It's just maths! What shape do you think the relationship has and why (eg flat the whole way and then a little increase?)?

I am indeed aware that we can be hit by a car when not in a car. This is an unavoidable risk in normal modern life. Yet I still continue to strap my children when I put them in a car.

And vaccines are one part of an overall development package to save lives. Clean water is indeed another part, together with antibiotics for pneumonia, and oral rehydration salts for example. They all contribute to the same thing - helping children survive and lead productive and healthy lives.
You may find this article of interest
saving children's lives

bumbleymummy Wed 12-Sep-12 22:23:17

Really seeker? I'm talking about women walking miles to get clean water for their children and you talk about Theodore Roosevelt. hmm

bumbleymummy Wed 12-Sep-12 22:26:02

I asked if you think that having even a few people immune (or a few less susceptibles if you prefer) means you have herd immunity. Is that what you think?

bumbleymummy Wed 12-Sep-12 22:29:12

You might find this interesting.

"1.1 billion people lack access to safe drinking water. 2.6 billion people lack adequate sanitation. 1.8 million people die every year from diarrhoeal diseases, including 90 % of children under 5 . "

ElaineBenes Wed 12-Sep-12 22:29:13

Why don't you tell me what you're getting at BM? I answered you above.

seeker Wed 12-Sep-12 22:29:16

I mentioned Theodore Rooseveldt because his case shows that yu can be rich and have access to clean water and sanitation and still end up in a wheelchair because of polio.

If anyone is belittling anyone you are being very dismissive of every child in the developing world killed or crippled by polio.

AnitaBlake Wed 12-Sep-12 22:29:33

Am confused? I worry about getting hit by a car every time I walk down a street. My 'immunisation' is that I know the green cross code. It doesn't mean I won't be hit by a car, but it certainly reduces my chances. Same as giving my baby vaccinations does stop them getting diseases but reduces their chances of being harmed by the disease in question.

Isn't that what all parents do? The best they can to reduce the chances of their children coming to harm?

AnitaBlake Wed 12-Sep-12 22:30:53

Doesn't stop them that should read, obviously!

ElaineBenes Wed 12-Sep-12 22:32:56

No one here has said that safe drinking water is a bad thing! It's not, it's an excellent thing and would indeed save many lives.

And in the Lancet article I linked to, it would save more lives than measles vaccines do.

Vaccines save lives. Clean water saves lives. Both are very good things and something people should be entitled to.

You started a thread on herd immunity and suddenly you want to discuss development priorities???? A little distraction technique going on here methinks.

ElaineBenes Wed 12-Sep-12 22:34:11

Sorry, you didn't. Tabitha did. But still, start a new thread on development priorities if you have an issue with poor countries spending money on vaccinating children. Don't try to derail this thread.

ElaineBenes Wed 12-Sep-12 22:36:23

Actually polio is a strange disease because the severity actually increases when you have access to clean water and aren't exposed to the disease as an infant.

bumbleymummy Wed 12-Sep-12 22:37:21

No you didn't EB. You said it would reduce the number of susceptibles. I said:

" You seem to talk about herd immunity as if it exists regardless of the number of susceptibles. Is that what you think? That as soon as you have a few immune people in a population then you have herd immunity?"

So is that that you think? That as soon as you have a few immune people/a few less susceptibles that you have herd immunity?

I am not being dismissive of them at all. I just think your comparison between us sitting here discussing polio when people are walking miles to get polio vaccines is inappropriate. Particularly when you consider how many millions of people are dying from lack of a something like clean water in comparison. If you want to get incandescent about something, get incandescent about that.

bumbleymummy Wed 12-Sep-12 22:39:15

Hopefully my last paragraph will explain why I went off in a clean water tangent. If you're going to make comparisons between developed and developing countries there are much more glaring differences than vaccine availability.

seeker Wed 12-Sep-12 22:41:26

"I am not being dismissive of them at all. I just think your comparison between us sitting here discussing polio when people are walking miles to get polio vaccines is inappropriate. Particularly when you consider how many millions of people are dying from lack of a something like clean water in comparison. If you want to get incandescent about something, get incandescent about that."

I genuinely don't understand. This thread is about immunity to disease. Not about clean water. I regularly do "get incandescent" about that as well. But that is not what this thread is about. Please will you stop with the smoke and mirrors and stick to the subject.

bumbleymummy Wed 12-Sep-12 22:45:16

Whatever seeker. Maybe we should stay clear of comparisons between developing and developed countries. To put it nicely, it's not exactly a 'fair' comparison.

seeker Wed 12-Sep-12 22:49:01

Why? Because you haven't actually got the insensitivity to tell mothers in developing countries that you don't think they should vaccinate their child against the illness that killed its siblings?

ElaineBenes Wed 12-Sep-12 22:51:59

(big sigh)

let's put it this way BM. If you reduce the number of susceptibles, you reduce the probability of transmission of disease. if you reduce them by a little, you reduce the probability by a little.

If you wish to call this 'herd immunity' or 'mathematical models of disease transmission', I don't care. If you have a few people vaccinated you have a little 'herd immunity' - the exact amount would depend on the relationship between the number of susceptibles and disesae transmission.

As I said, I suspect it's S shaped so you probably won't have much of an effect at low levels of vaccination.

It is not a belief system - it's a mathematical model.

bumbleymummy Wed 12-Sep-12 22:58:16

The vaccine won't be much use if the child starves to death or dies from drinking contaminated water will it? See how inappropriate you're being? Leave it.

seeker Wed 12-Sep-12 23:03:46

No I won't leave it. You are being ridiculous. And you are diminishing the death of every child who dies of a preventable disease in the developing world. It is shameful that not everyone has access to clean water. Shameful. And it is also, and separately, shameful that children wre not protected from preventable diseases.

bumbleymummy Wed 12-Sep-12 23:05:26

EB, your own link talks about reducing the number of susceptible people below a certain threshold in order to achieve herd immunity. You do not have 'a little herd immunity' because you vaccinate a few people. You either have it, because a significant proportion of the population is immune, or you don't.

ElaineBenes Wed 12-Sep-12 23:06:12

if you're genuinely interested in modelling the impact on mortality of different interventions (which unfortunately do have to take into account mortality from other causes), you can download this software

LIST

It's actually an important question and one that development professionals consider very carefully - what's the most cost-effective tool for saving lives?

ElaineBenes Wed 12-Sep-12 23:08:09

Like I said, I think it's an S shaped relationship. You can indeed have a little 'herd immunity', just not enough to have a significant effect.

The theshold would be the bend in the S. I had a feeling that was what you were getting at. I wish you'd just say, it'd make life easier.

ElaineBenes Wed 12-Sep-12 23:11:10

This paper shows the potential impact of even quite low levels of vaccination on herd immunity.
www.ncbi.nlm.nih.gov/pubmed/22778297

bumbleymummy Wed 12-Sep-12 23:11:59

I'm being ridiculous? You're comparing us with people living without basic access to food, water,sanitation and healthcare to try to suggest we take vaccines for granted and you think it's an appropriate, valid comparison. hmm Why not take away our access to water, food, sanitation and healthcare and see how we fare even with our vaccines.

bumbleymummy Wed 12-Sep-12 23:13:41

Not according. The definition Elaine. You can be 'on your way to achieving' herd immunity because you are reducing the number of susceptibles but you do not 'have a little' of it.

bumbleymummy Wed 12-Sep-12 23:17:02

I'm sure you're aware that the threshold for herd immunity is different for various diseases EB.

seeker Wed 12-Sep-12 23:18:48

Bumblymummy, appear to have misunderstood me. Maybe if you have another read of my post, you might understand it better.

"I get really incandescent with rage sometimes when I think of mothers in the developing world carrying their children for bloody miles in the hope of vaccinations which might actually give them a chance of reaching adulthood, while us comfortable Westerners burble on about polio being nothing much to worry about."

The "us" in the penultimate line is a mere courtesy- I of course meant you and your fellow travellers.

ElaineBenes Wed 12-Sep-12 23:23:12

Yes, I am aware that the herd immunity threshold is different for different diseases depending on their parameters.

As has been explained to you before, there is a threshold at which point the disease cannot be sustained in the population. This is what is being referred to as the herd immunity threshold.

Below this threshold, the level of immunity in the population will affect the probability of an epidemic, the time between epidemics and the severity of an epidemic. You can indeed have a little herd immunity where the effect is small. You can have a lot of herd immunity where the effect is great.

I think you're intentionally using the term 'herd immunity' in different ways to confuse.

bumbleymummy Wed 12-Sep-12 23:26:24

Not sure what that has to do with my last post seeker hmm

bumbleymummy Wed 12-Sep-12 23:35:40

No, I'm not Elaine. You are the one introducing the idea of 'a little herd immunity'.

ElaineBenes Wed 12-Sep-12 23:44:38

OK, whatever you want to call it - I don't care. But the point is there is no magic threshold where suddenly having less susceptibles doesn't matter to the probability of disease transmission.

There IS a threshold where the disease can not be sustained in the population. How far you are below that threshold impacts upon the probability of an epidemic starting (ie the average time between epidemics) and the severity of the epidemic (ie the peak). The more who are immune the more time you have between epidemics and the less severe they are, on average.

Other than semantics, what don't you agree with?

sashh Thu 13-Sep-12 03:46:23

Elaine, in order for a disease to become stablein the community, R must be <= 1. If it is greater than 1 then the disease will continue to spead until it again reaches a point where R=1

How can R>1 rise to R=1

ElaineBenes Thu 13-Sep-12 04:00:07

Because the susceptibles becomes removes either through dying or becoming immune (either through exposure to the disease or through vaccination).

R is a function of the density of susceptibles amongst other things, so everything else being equal if everyone around you has either died or become immune, you will infect fewer people.

mathanxiety Thu 13-Sep-12 05:38:55

'BBB You describe polio as being very serious, yet I've only met one person who suffered permanent damage from it, out of all the pensioners I know and have met in the past.'

An old schoolmate of mine survived polio and lived her short life confined to a wheelchair. You met the lucky ones who lived to be pensioners. A lot of people who had polio in their youth end up dying relatively early of respiratory problems or become housebound because of muscular or joint problems that rob them of their mobility (post polio syndrome).

There is no way to predict what victims will develop paralysis. There is no cure. Victims either live or die, and adapt as best they can to wasted muscles or whatever other after effects they are left with.

Polio victim

Another here -- RC Archbishop of Chicago, walks with a pronounced limp, normally uses a cane.

Polio victims in iron lungs, 1950s USA.

Polio victims in leg braces.

Polio victims here and here who don't have the benefit of leg braces.

The iron lungs were respirators. Without them many patients would have suffocated or died of pneumonia, as polio causes paralysis and often the lungs were affected. They were also used for diphtheria patients.

Interesting blog and comments.

mathanxiety Thu 13-Sep-12 06:09:46

'The vaccine won't be much use if the child starves to death or dies from drinking contaminated water will it?'

We're all going to wind up pushing up daisies some time, so why bother with anything that enhances health, if that's going to be your attitude?

'Why not take away our access to water, food, sanitation and healthcare and see how we fare even with our vaccines.'

I don't think anyone would argue with the proposition that in the 1950s there was no more developed country than the USA, and yet before the polio vaccine, children and adults died from this disease by the thousands.

Ironically, it could have been better hygiene that left children without the sort of exposure that might have given them a chance to develop immunity. It is perhaps partly because of our better hygiene that we need vaccinations.

Of course it could be argued that we should ignore hygiene and our children would then develop immunity or become sick and die. After all, there are a lot of people around today who had measles or mumps and survived. The question is do you want your child to be the one in ten thousand or one thousand who doesn't make it. You can't tell just by looking at them who will pull through and who won't. And you can't tell what will happen to a child your child infects.

LeBFG Thu 13-Sep-12 07:04:49

"plenty of people seem to keep saying 'it's not all or nothing'" - that's because it is NOT all or mothing!

"as if the threshold doesn't even come into it." of course this does come into it. That's why they were panicking over the MMR. It was pretty much at threshold levels just before the Wakefield report and then plummeted.

"'magically protected' no. Less likely to contract and/or be seriously damaged by them due to" .... meeting people that are vaccinated/immune! Simple. As I said.

I don't need to repeat Elaine's posts to explain herd immunity. You are using the term differently. If you want to get technical - you can describe herd immunity as the proportion of immune people. This can be naturally-acquired immunity or vaccine-acquired (though most people mostly use this in context of vaccination rates). You can describe what people on here (including me) have described as the herd immunity effect, or herd effect as how the spread of disease is impacted by the proportion of vaccinated/immune people.

Of COURSE herd effect is in operation with vaccinated populations. Herd effects happened in the pre-vaccination era too - the problem there was in years between outbreaks, disease transmission was lower, less immune individuals (lower herd immunity), reduced herd immunity effect (reduced herd effect), populations became susceptible to outbreaks. Nowadays we have these wonderful things called vaccines which maintains herd immunity (thus herd effect) without even needing to get the disease! What more could you ask for? High proportion of immune individuals AND lowered risk of getting disease if you're not immune. Science in a wonderful thing.

bumbleymummy Thu 13-Sep-12 13:32:58

EB, what I want to call it? It has nothing to do with what I want to call it. It's the definition of what it is. As you've pointed out yourself, it is a mathematical model. The result is either true or false, it can't just be 'a little bit true'. Either there is a significant proportion of the population to allow the disease to become stable or there isn't.

With natural herd immunity, susceptibles are removed from the pool for life because natural infection (in the vast majority of cases for most diseases) will confer life long protection. The thresholds for natural herd immunity are much lower than those recommended for vaccination. With vaccines, not only do you have the vaccinated non-immune left in the pool but then the ones who initially were immune end up back in the pool due to waning immunity. It means that achieving 95% (the threshold for many of the diseases) vaccination coverage doesn't necessarily mean that you have achieved herd immunity because you don't know the immune status of the rest of the population. In the case of whooping cough, having 95% uptake of the vaccine (it may actually be 96% at the moment) means very little because the immunity wanes so quickly.

LeBFG, see above.

Also, you mentioned about the evidence of herd effect wrt pertussis is in the reduction of the deaths children too young to be vaccinated but if you look at figure 1 On page 8 of this you can see that the number of deaths from pertussis in children < 2 months (ie under the age of vaccination) has increased since the introduction of vaccination.

bruffin Thu 13-Sep-12 13:52:42

On page 8 of this you can see that the number of deaths from pertussis in children < 2 months (ie under the age of vaccination) has increased since the introduction of vaccination
It doesnt say that at all. It is graph based on % of the age children die of WC in the first year.
It doesnt say how may deaths there are in the first place.
There are now a higher % of deaths in the first two months because babies are vaccinated against wc at 2 months and are unlikely to get it after that and therefore not die.
It is not comparing the numbers of deaths at all.

bruffin Thu 13-Sep-12 14:07:08

A clearer way of putting it

The graph represents the deaths of children in the first year of their life,comparing them over 3 different time periods. It does not say how many children died in each of those periods.

The graph is then broken down on a monthly basis to show what % of those deaths happened in the first, second, third month of life etc.

Most of the deaths in the earlier period were more evenly spread, in the latest period they are concentrated when children are unvaccinated.

bruffin Thu 13-Sep-12 14:25:41

By the way
the white blocks (1938-1940) represented 7123 deaths,
the light blue block 1990-1999 represented 94 deaths
and the dark blue blocks2000-2006 represented 145 deaths.

Vaccines really dont work do they hmm

bruffin Thu 13-Sep-12 14:26:19

that was supposed to be a hmm

LeBFG Thu 13-Sep-12 14:30:02

I haven't the appropriate knowledge or experience of the field to comment on the figure. Just a random thought. They always say premature babies are more susceptible to wc death and modern medicine has improved so much since the 30's such that premature survival rates have increased considerably over the same period. Perhaps this explains why 1-2 month olds may be more vulnerable?

Herd immunity threshold for wc is 92-94%. If you were talking about immunity waning in months I would agree with you, but the estimated duration is 4-12 years. Natural immunity at 4-20. There is a great amount of variation between individuals. As I've said, regardless of what percentage of people really are immune, the protective herd immunity effect of wc vaccination is measurable, and strikingly so, and repeatable in many populations in many countries. Look it up.

ElaineBenes Thu 13-Sep-12 15:00:20

And even if the number of people immune to wc in a population is not sufficient to stop the disease spreading, having more immune people through vaccination means that any epidemic/outbreak of wc will be shorter with longer intervals between outbreaks than if you didn't vaccinate thus reducing the lifetime probability of any non-immune individual contracting wc. This is the point that BM seems to be missing.

ElaineBenes Thu 13-Sep-12 15:01:37

BTW, BM, you yourself said that there are differing definitions of herd immunity so it certainly is about what you want to call it. Pick one, stick to it, and I'll work with you with whichever you choose. I really don't care.

LeBFG Thu 13-Sep-12 15:12:25

Better expressed again than my post Elaine.

I have to alter the wording of my post. When I say "regardless of what percentage of people really are immune..." - I mean to say, "whatever the percentage of immune people at high vaccination rates...."

Whether coverage is at threshold or not (though, it would ideally be at or above this level), a protective effect of herd immunity is still in action at current vaccination rates (though would be at it's highest at threshold of course).

ElaineBenes Thu 13-Sep-12 15:17:40

It seems however we may express this very basic notion, if someone doesn't want to understand because it doesn't fit their world view, they won't.

seeker Thu 13-Sep-12 15:52:20

I think it's classic diversionary tactics. Debate tw meaning of herd immunity- accuse someone of not caring about the issue of fresh water in the developing world- anything rather than address the actual issue

bruffin Thu 13-Sep-12 15:55:19

I haven't the appropriate knowledge or experience of the field to comment on the figure. Just a random thought. They always say premature babies are more susceptible to wc death and modern medicine has improved so much since the 30's such that premature survival rates have increased considerably over the same period. Perhaps this explains why 1-2 month olds may be more vulnerable?

BFG, BM misrepresented the graph, it is not comparing the amount of deaths of babies in the various decades.
In fact the graph is a superb illustration of why herd immunity from vaccination works. Even the deaths of the most vulnerable (ie too young to be vacccinated) are a fraction of what they were before were before we started vaccinating.

LeBFG Thu 13-Sep-12 16:13:12

Yes, you're right bruffin. I wonder what bm'll make of it? I suppose my point stands, if not in numbers but proportions. Proportionately younger babies are more at risk of dying of wc, even though number of deaths have dropped?

Seeker, this is why is can be so depressing debating with the ilk of bm. How is it possible to move things forward?

And Elaine: what I can't get my head around is what exactly is bm's world view? What is she trying to argue here? That herd immunity has no effect unless it is over a threshold value?

ElaineBenes Thu 13-Sep-12 16:17:40

Yes, leBFG. Because then you can say that it doesn't matter for others if you don't vaccinate your children since we're not at 'the threshold'. It's a guilt assuaging thing imo. Anti-vaxers like to believe that vaccine induced herd immunity is a myth (but love how it works no problem with CP vaccination because the impact of herd immunity with CP is to increase incidence of shingles)

bumbleymummy Thu 13-Sep-12 18:22:15

There are different definitions of herd immunity, I've linked to/quoted some of them but none of them allow this idea of there being 'a little herd immunity'. The most popular/widely accepted meaning on this thread seems to be that when a certain proportion of the population are immune, the disease becomes stable within the population. The thing that some of you seem to be unable to accept is that this is a mathematical concept. When the threshold is reached, there is herd immunity, if it is not, then there is not. You can talk about reducing susceptibles all you want but you can't define it as 'herd immunity' until you reach that threshold. R <= 1 is either true or false. There is no 'a little true'.

Apologies for any confusion about the graph. It does actually show an increase in the proportion of deaths in infants under 2 months.

JoTheHot Thu 13-Sep-12 18:47:03

bm, I'm lost as to how you find such simple concepts so confusing. Here's your first quote, which accurately explains that

"The term herd immunity has been used by various authors to conform to different definitions..... We propose that it should have precise meaning ...: "the proportion of subjects with immunity in a given population"

It is very clear, and totally unarguable, that the proportion of immune subjects in a population can be anything between zero and one. It can be 'little', large or anything in between..

bumbleymummy Thu 13-Sep-12 19:01:18

Yes, that was their proposed definition and only one example given. That is not the context in which people are using it on this thread. In that example, the definition they used to describe what most people are talking about on this thread is 'herd effect'.

bumbleymummy Thu 13-Sep-12 19:05:10

Term* they use to describe

ElaineBenes Thu 13-Sep-12 19:05:46

Well. here's another:
definition of herd immunity

Personally, this definition and the one in your link is EXACTLY what I am referring to. I know you don't like it because you can no longer say 'Oh, there's no such thing as herd immunity below x% immunized' but that shouldn't be the reason for choosing a certain definition.

bumbleymummy Thu 13-Sep-12 19:14:50

From your link:

"in which a significant proportion of the individuals are immune"

JoTheHot Thu 13-Sep-12 19:16:45

you said none of the definitions you had linked to allowed for herd immunity to be small. This was an untrue statement.

If a small proportion of people are immune, i.e. herd immunity is small, the herd immunity effect, often measured as the immunity-related reduction in the speed of spread of the disease, relative to the speed of spread in a naive population, will also be small. It is clear, and totally unarguable that the reduction in speed of spread of a disease due to immunity may be small, large, or anywhere in between.

You really are on a hiding to nothing here bm. Come back when you've got the maths straight.

ElaineBenes Thu 13-Sep-12 19:22:24

As I said, BM, I'd guess that the relationship is S shaped (a logistic function). So at very low levels of vaccination I wouldn't expect the effect to be significant (similarly the marginal effect at very high levels would be small). As the levels increase, the effect on transmission increases disproportionately to the middle of the curve where it starts to decrease.

I agree with Jo, BM. Either you're wilfully not understanding or you need to brush up on your maths (especially probability functions).

LeBFG Thu 13-Sep-12 19:26:14

"'The most popular/widely accepted meaning on this thread seems to be that when a certain proportion of the population are immune, the disease becomes stable within the population." I completely disagree that the thread has agreed to this defintion of herd immunity as it is clearly referring to HERD IMMUNITY THRESHOLD

"When the threshold is reached, there is herd immunity" - you are talking about the herd immunity EFFECT here. The threshold only tells us about whether a disease spreads or not.

Here's a simple example. Half of people in a population are immune to an infectious disease. Of the 10 people you mix with 5 are immune and 5 are potential sources of infection. If all 10 were not immune, every encounter with an ill friend could render you ill. When herd immunity is at 50%, you are exposing yourself to less ill people. You may avoid ever becoming ill, though your chances are much less if 90% of people you encounter are immune. This is why the EFFECT of herd immunity is still measurable under threshold levels.

As Elaine says, the particular profile will vary depending on many factors I can't think of (how contagious/rate of transmission of the disease is a pretty obvious one).

bumbleymummy Thu 13-Sep-12 19:27:50

Well Jo, earlier you argued that herd immunity only had one meaning. Which one were you thinking of?

I said none of them allowed for there to be ' a little herd immunity'. In your example, the herd immunity effect can not be small. It either exists or it doesn't - as the math shows.

Are you seriously trying to argue the the statement R <= 1 can be 'a little bit' true. Really?

bumbleymummy Thu 13-Sep-12 19:29:25

No BFG, the herd immunity threshold is the percentage of the population that need to be immune in order for the disease to become stable in the community.

LeBFG Thu 13-Sep-12 19:57:47

Spot the difference:

percentage of the population that need to be immune in order for the disease to become stable in the community

a certain proportion of the population are immune, the disease becomes stable within the population

bumbleymummy Thu 13-Sep-12 20:11:42

You left out a bit from the second quote leBFG - '^when a^
certain proportion of the population are immune, the disease becomes stable within the population.' The 'certain proportion' refers to the herd immunity threshold, the state of being at or above that threshold is herd immunity/herd effect.

JoTheHot Thu 13-Sep-12 20:21:52

The herd immunity effect is the immunity-related reduction in R. Consider a case where R is 3 in a naive population (zero immunity), and 2.9 in a population with 10% immunity. The herd immunity effect, R reduced from 3 to 2.9, is small. This is very clear and totally irrefutable.

At some level of population immunity, the herd immunity effect is sufficient that R falls to 1. This level is called the herd immunity threshold.

Given the sheer volume of total bollocks you've disseminated on vaccine threads, I'm finding it not inconsiderably entertaining to find you demonstrably and unambiguously flailing around, out of your intellectual depth, on a topic core to understanding vaccination.

Tabitha8 Thu 13-Sep-12 20:34:06

Oh come on, I'm the one bumbling about on this topic, not Bumbley hence why I started the thread.

ElaineBenes Thu 13-Sep-12 20:53:15

I really have got to stop as this is taking up so much time with so little reward!

BUt here is another description of the process
basic epidemic theory

They define herd immunity as simply that 'Vaccinating an individual indirectly
reduces the risk of infection of this individual’s contacts'

The 'threshold' they call 'Critical'vaccination'coverage'. ie The minimal proportion
needed to be vaccinated in order to prevent an epidemic (ie bring R down to below 1)

BM Slide 10 models for you the relationship between vaccination coverage, R0 and infection attack rates. As you can see, there is no magic point where the number of immune suddenly matters. It's simply that there is a point where an epidemic can be sustained and one where it will die out.

A bit like population growth - when each woman replaces herself with more daughters, a population will grow. When each woman replaces herself with fewer than one daughters (on average) a population will die out. A population is either growing or it's not. But the RATE of growth (or decline) is very important. It matters a lot if your fertility rate is 1 child per woman or 2 children per woman even though both are below replacement levels and the population is declining and both will eventually die out. It's the amount of time it takes which is also important.

bumbleymummy Thu 13-Sep-12 22:00:57

Jo, Herd effect is the reduction of disease in the unimmunised population as a result of immunising a proportion of the population.

R is the effective reproductive rate. It is the basic reproductive rate (R0) discounted by the proportion of the population that is susceptible (x). Eg. If R0 is 10 but only half the population are susceptible (x =0.5) then R = 5 (10x0.5). In order for the disease to become stable in the community then R needs to = 1 (if it is < 1 then the disease will die out).

The 'immunity-related reduction' in the above example is x. x is not the herd effect. X is the proportion of the population who are not immune.

Elaine, you find one line on one slide from one presentation by one author that says that and that negates all the other definitions/links that have been given here? The 'magic point' is the herd immunity threshold ie the point where the disease will become stable or, if the threshold is exceeded, will die out.

ElaineBenes Thu 13-Sep-12 22:36:04

Actually their definition fits in nicely with what everyone else is saying....apart from you.

What is wrong with that definition apart from the fact that you don't like it?

You're also wrong speaking about a disease being 'stable'. A disease will only be stable when R = 1 exactly. In the same way that a population will only be stable when the net reproduction rate = 1 (ie taking into account mortality, each woman produces one daughter on average). When R > 1, a disease will spread - ie an epidemic. When R < 1, a disease will die out. If you can sustain R < 1 through reducing the number of susceptibles in a sustained manner, you can eradicate a disease in a population. This is the herd immunity threshold. R will fluctuate as the balance between susceptibles/infected/removes fluctuates.

However, the point I am making which you are failing to address either because you don't understand or because it doesn't fit with your world view is that below that threshold, it matters A LOT to disease transmission if R = 1.1 or R = 3. If by vaccinating, you can reduce R from 3 to 1.1, even if you are above the threshold (ie a disease will spread), you will have longer between epidemics and the severity of an epidemic will be reduced. This is shown in slide 10 of the presentation I linked to. Therefore, any non immune individual's probability of contracting a disease will be affected by the number of vaccinated people in the population, even if the number of immune people is not sufficient to stop a disease spreading.

Whooping cough is a good case in point. In other words, any one individual choosing not to vaccinate their children has an impact on the probability of other children getting the disease in question even if vaccination levels are not high enough to completely prevent outbreaks. I know you don't like this fact and this is why you are pretending that it's not true and using a rather frustrating smoke and mirrors technique around definitions - but you unfortunately you can't argue with mathematical realities.

ElaineBenes Thu 13-Sep-12 22:39:48

This thread reminds me of that scene from Monty Python's "The Search for the Holy Grail" with the Black Knight who refuses to stop fighting even when he's had all his limbs cut off!

JoTheHot Fri 14-Sep-12 07:48:47

"The 'immunity-related reduction' in the above example is x. x is not the herd effect. X is the proportion of the population who are not immune."

It's no mean feat to contradict yourself within so few words. x can not simultaneously be both the reduction in R (statement 1) and the proportion of unimmune people (statement 3).

The herd immunity effect in my example is [R(0)-R(0.1)]/R(0) = [3.0-2.9]/3.0 = 3%.

Thanks for the black knight Elaine. It's good to start the day with a chuckle.

seeker Fri 14-Sep-12 08:13:45

So, bumblymummy- should children be vaccinated against polio or not? Yes or no.

bumbleymummy Fri 14-Sep-12 10:30:24

Elaine, it does not fit in with most of the other definitions that have been posted. Can you really not see that?

Wrt herd immunity threshold. If the threshold is reached then the disease becomes stable (R=1) , if the threshold is exceeded then the disease will die out. (R <1) Are you disagreeing with that?

I have said several times that you can talk all you want about decreasing susceptibles and reducing transmission but that is not the same as having 'a little herd immunity'. You either have it or you don't. A significant proportion of the population need to be immune in order for herd immunity/herd effect to exist.You may argue that 3 immune people in a population of 1,000 may slightly reduce transmission but are you actually going to argue that they provide 'a little herd immunity'?

Yet another link that agrees with what I'm saying

"In the illustration below, the top box depicts a community in which no one is immunized and an outbreak occurs. In the middle box, some of the population is immunized but not enough to confer community immunity. In the bottom box, a critical portion of the population is immunized, protecting most community members."

bumbleymummy Fri 14-Sep-12 11:13:58

Jo, note the quote marks.

Also, what are you doing with your calculations?

In your example, if R = 3 in a population with no immune then R0 = 3.

If 10% are immune then the fraction of the population who are susceptible( x) is 0.9.

R = R0x so, in your example, R = 2.7 not 2.9.

Maybe not so 'clear and totally irrefutable' as you thought...

The herd immunity threshold in your example would be 66%. Ie 66% of the population would need to be immune for the disease to become stable in the community. If that threshold was exceeded then the disease would die out.

ElaineBenes Fri 14-Sep-12 11:51:19

What would be the difference on disease transmission between an R of 1.1 and an R of 3?

Actually your calculation above just makes the point further! You have further reduced R by immunising.

What is the effect on disease transmission of reducing R? does it matter it R = 1.1 or if R =3? You keep on avoiding this question and just saying that a disease will die out if R < 1. We know that, there is no disagreement on this issue.

ElaineBenes Fri 14-Sep-12 11:55:49

And you most certainly can have a little herd immunity depending on how you define it.

I'm afraid that you are not the keeper of definitions of herd immunity bumbley. You can define it as the threshold, I prefer to use other definitions which I think are clearer.

bumbleymummy Fri 14-Sep-12 12:07:45

I'm not avoiding anything Elaine, you just keep talking about reducing susceptibles/transmission as if any reduction can be classified as herd immunity despite the fact that pretty much every definition of herd immunity/community immunity has said that a significant proportion are required to have that effect. Do you think having 3 immune people in a group of 1,000 would be classed as herd/community immunity?

seeker Fri 14-Sep-12 12:13:17

Bumblymummy, do you think children should be vaccinated against polio or not? Yes or no?

bumbleymummy Fri 14-Sep-12 12:22:12

Ok then, define it Elaine. (as I asked way back at the start) What are you talking about when you are using the term 'herd immunity'. Are you referring to the proportion of the population who are immune or are you talking about the protective effect that can be conferred when a significant proportion of the population are immune?

bumbleymummy Fri 14-Sep-12 12:24:26

seeker, it has nothing to do with me whether other people vaccinate their children against polio or not. I don't mind whether they do or not. What does that have to do with this thread? hmm

seeker Fri 14-Sep-12 12:34:37

It has do with the thread in two distinct ways.

1)Do you think your children would be safe from polio in an unvaccinated community?
And

2) It is customary in a debate to declare an interest. If you are an anti vaccination campaigner then it would be courteous to be open about it.

bumbleymummy Fri 14-Sep-12 12:38:56

Do I think they'd be 'safe' from contracting it in an unvaccinated community? No, why would they be? They'd have the same chance of catching it as anyone else.

<snort> at anti-vaccine campaigner. No, I'm not.

seeker Fri 14-Sep-12 12:45:16

Ah. And polio isn't something to fear, then?

LeBFG Fri 14-Sep-12 12:49:14

...with the exception that anytime she can, bm is on these threads dismissing vaccines. Unrelentingly. The only positive thing I've ever seen her mention is that inhalable vaccines look 'interesting'. I have a strong suspicion that with the advent and rolling out of these new-style vaccines, bm'll find good reason to diss these too.

bumbleymummy Fri 14-Sep-12 12:51:32

Wow, you have a short term memory BFG. We spent quite a lot of time discussing vaccinating older children/adults against things like rubella and CP not too long ago.

seeker Fri 14-Sep-12 12:52:46

And don't forget the mysterious aluminium issue.

bumbleymummy Fri 14-Sep-12 12:52:49

seeker, I guess you could fear it if you wanted too.

bumbleymummy Fri 14-Sep-12 12:54:45

To*

seeker Fri 14-Sep-12 12:55:15

Bm- I am really wondering why you won't answer straight questions with straight answers.

Yes I would fear polio. Because I am old enough to remember when there were epidemics of it.

Would you?

ElaineBenes Fri 14-Sep-12 13:03:57

If you're not avoiding my question bm, then pls answer. you clearly are avoiding it!

Do you think it matters for disease transmission if R is 3 or 1.1?

LeBFG Fri 14-Sep-12 13:10:51

I think you should get on with anwering those two questions first bm.

With my short term memory I retained two things:

wrt rubella: you proposed no one (not even siblings) should be vaccinated to protect someone else. You do think it OK however for women of childbearing age should check immunity regularly until menopause and before she decides to become pregnant, vaccinate then.

wrt CP: again, vaccinating only women if they want children, after testing for immunity.

My longer term memory (this is fuzzier) says you are OK with vaccinating against a terrible disease (not sure which one, it might have even been hypothetical) only if you come directly into contact with an ill person.

I suppose I disregard these things as I feel they are so unworkable as to not really count as being Ok about some vaccinations. It's a bit like me saying I've nothing against football on Saturday's in months with 29 days...oh, and only between 12-2pm.

bumbleymummy Fri 14-Sep-12 13:14:26

No, I wouldn't.

I bet there are things that I fear that you don't though. Everyone is different.

EB, you're still avoiding mine. Yes, I think it matters. Can you answer my questions now?

Do you think having 3 immune people in a group of 1,000 would be classed as herd/community immunity?

What are you talking about when you are using the term 'herd immunity'. Are you referring to the proportion of the population who are immune or are you talking about the protective effect that can be conferred when a significant proportion of the population are immune?

bumbleymummy Fri 14-Sep-12 13:20:21

I'm not sure how someone can be considered 'anti vaccine' if they suggest it even for certain groups. hmm I'm pretty sure people who are genuinely anti-vaccine wouldn't agree with that but hey-ho.

Yes, your short term memory is poor.

LeBFG Fri 14-Sep-12 13:45:14

You're not anti-vax - I'm not anti-football either.

Tempernillo Fri 14-Sep-12 13:48:13

People are vaccinated in childhood, so they still have immunity in adulthood most of the time. Children are more vulnerable to childhood illnesses. Their immune system is less developed, they are more vulnerable to the effects of the disease, and they mix with close contact at school. That is why vaccination programmes ate aimed at children.

seeker Fri 14-Sep-12 14:19:18

So, bm. Imagine a hypothetical situation where you live in an area where polio is endemic and nobody's vaccinated. There are cases in your child's school. Are you saying that you would be perfectly happy with your children going to school? Because this what parents in some parts of Africa have to do. And you were completely dismissive of their efforts to get the vaccine for their children and said they should concentrate their efforts on getting water.

bumbleymummy Fri 14-Sep-12 14:29:40

Actually tempernillo, most childhood diseases are more risky in adulthood. Pushing the disease into adulthood is actually recognised as a perverse effect of vaccination.

Seeker, I didn't say that they should 'concentrate their efforts on getting water' hmm I do still think that you should move away from this comparison with Africa though. Polio may be quite far down their list as far as worries about their children go. As I've said before, it's not really a fair comparison.

seeker Fri 14-Sep-12 14:34:04

Vaccination is actually quite high up the list of priorities of people in the developing countries. It is a way of protecting the children they have now.

The comparison is entirely valid. If you expect a mother in the developing world to send her unvaccinated child to school during a polio outbreak, the least you can do is say whether you would do the same.

Answer the question.

mathanxiety Fri 14-Sep-12 15:31:49

Different diseases have different thresholds. Measles requires over 90% immunity threshold for an outbreak to be contained. Smallpox requires somewhere around 80%.

mathanxiety Fri 14-Sep-12 15:34:04

'Pushing the disease into adulthood is actually recognised as a perverse effect of vaccination.'

...but remember that with herd immunity, that effect is avoidable if enough people in a population are vaccinated (and boosters are available).

ElaineBenes Fri 14-Sep-12 16:01:42

'Pushing the disease into adulthood is actually recognised as a perverse effect of vaccination.'

for the unvaccinated of course....

But as math said, if you can sustain herd immunity at high enough levels, even unvaccinated people have a low lifetime probability of being exposed to the disease - and eventually you may even eradicate the disease in question.

minceorotherwise Fri 14-Sep-12 17:08:34

Have we ever achieved herd immunity to measles in this country?

ElaineBenes Fri 14-Sep-12 17:25:21

I'm assuming, mince, that you mean herd immunity past the threshold where the disease can be sustained in the population? We've always had some herd immunity given the levels of measles vaccination.

I think there are areas where herd immunity is strong and areas where it is weak (as we don't all mix randomly throughout the whole country)

areas according to MMR vax rates

So you're pretty safe that there won't be a measles epidemic in Barnsley but herd immunity isn't sufficient to prevent an epidemic in many parts of London.

LeBFG Fri 14-Sep-12 17:42:55

Yes Elaine, I've seen the breakdown on vaccination rates by area - they appear consistently depressed in London wrt to lots of vaccines. Why would that be?

bumbleymummy Fri 14-Sep-12 18:19:06

Seeker, You're asking the same question over and over in different ways. How many variations of the same question are you going to ask? What point are you trying to make? Yes, I would.

Math, who is your post about different thresholds directed to? It's been mentioned a few times on the thread.

Wrt 'boosters being available' - it's all very well them being available but people would actually need to get them otherwise you're just left with a lot of susceptible adults with waned immunity.

EB, you still haven't answered my questions even though I answered yours. I think mince would probably appreciate the clarification on what you are defining herd immunity as. Here they are again:

Do you think having 3 immune people in a group of 1,000 would be classed as herd/community immunity?

What are you talking about when you are using the term 'herd immunity'. Are you referring to the proportion of the population who are immune or are you talking about the protective effect that can be conferred when a significant proportion of the population are immune?

"for the unvaccinated of course...."

No, not just for the unvaccinated. Also for those whose immunity gas waned in adulthood.

Your 'if' wrt achieving a significant proportion of immune people in the population is an important one.

ElaineBenes Fri 14-Sep-12 18:22:46

asked and answered way way back in the thread bm

mathanxiety Fri 14-Sep-12 18:29:42

For undervaccinated populations (children receiving some but not all jabs, children receiving partial series) the reason tends to be faults in the healthcare delivery system.
For unvaccinated populations (children not receiving any) the reasons tend to be parental attitudes and education, and family circumstances (new migrants for instance).

Wrt parental attitudes -- In Ireland MMR vax rates suffered a dip in uptake after the Wakefield theory was advanced.

seeker Fri 14-Sep-12 18:35:28

Great, bm. A straight answer at last.

Now will you tell me why you would expose your children to polio- do you think they are not going to get it? Do you think if they do there is a cure? Or that they are not at risk of death or paralysis?

mathanxiety Fri 14-Sep-12 18:45:00

'Do you think having 3 immune people in a group of 1,000 would be classed as herd/community immunity?'

I suppose my statement about different diseases requiring different thresholds was addressed to you, BM, since you keep on asking this question and apparently not differentiating among diseases when you ask.

In the case of HIV/AIDS for instance (yes, std so not quite the same infection pathway as droplet infection type diseases) -- if back in the early stages of the epidemic everyone who was engaging in the behaviours associated with the spread of HIV had used condoms every time they had sex then the disease would not have spread. Maybe three people out of a community of one thousand using proper prophylaxis would have been sufficient to contain the outbreak? (Assuming three gay men in a community, one of whom ventures out of the community for R&R every so often and one of those three men also has a female partner who is two timing him with the postman, and the third has an on and off relationship with the one who plays away [R&R man]...)

In the case of measles, which is spread by contact with infected fluids from nose or mouth and by aerosol transmission too (sneezing and coughing), with a guarantee that about 90% of unvaccinated people who come in contact with a case will get the disease, and since an infected person will be infectious before the characteristic rash appears so staying home when you're sick is not possible, herd immunity (which is not a guarantee that an individual will not get measles) kicks in when over 90% of people are vaccinated.

JoTheHot Fri 14-Sep-12 19:04:21

So bm, you reckon can calculate the herd immunity threshold from R(0) using the formula

Herd immunity threshold = 1 - [1/R(0)]

You use this formula to question my calculations. Unfortunately, this formula gives the wrong herd immunity threshold for every single disease in this table.

I have my own little idea as to why: this formula is a crass simplification you've found on an epidemiology for dummies webpage. I'm sure you have another explanation. Perhaps you'd like to share it with us all?

Tabitha8 Fri 14-Sep-12 19:04:55

If measles vaccination rates in London aren't high enough to prevent epidemics, why is there no epidemic? Where are the measles cases? There was a lot of talk in the press, last year I think, about measles spreading across the country, but it didn't happen. Why not?

ElaineBenes Fri 14-Sep-12 19:06:52

I think the point is, math, that below that threshold since you can still dent the time between epidemics and the severity of an epidemic by having more people immune. The disease may spread but the rate of spread will be lower and the transmission chain shorter and the build up of suscpetible people in a population between epidemics will be slower, meaning that there will be more time between epidemics. I think it's a very important point because people who don't vaccinate say 'oh well, it doesn't matter because we don't have herd immunity anyway' - but it matters a lot!

ElaineBenes Fri 14-Sep-12 19:09:05

First of all, Tabitha, there have been measles outbreaks in London - these are essentially small epidemics. Why have they been small? Because most people do still vaccinate. Why have they happened? Because not enough people vaccinate to prevent them.

This is exactly what is expected to happen based on the theory of herd immunity.

measles cases in London

Tabitha8 Fri 14-Sep-12 19:19:07

Oh dear. Do I now need to worry about the definition of epidemic as well? Can we not use a layman's term? To me it means what was implied last year. That measles would spread across the country.
Several small outbreaks should combine to form one epidemic, should they not?

seeker Fri 14-Sep-12 19:23:49

If it spreads across the country it's a pandemic. Epidemic means substantially more cases than expected.
This from wiki- in the hopes of preventing derailment by definition!
"In epidemiology, an epidemic (&#949;&#960;&#943; (epi)- meaning "upon or above" and &#948;&#942;&#956;&#959;&#962; (demos)- meaning "people"), occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience.[1]:354[2] Epidemiologists often consider the term outbreak to be synonymous to epidemic, but the general public typically perceives outbreaks to be more local and less serious than epidemics[2][1]:55, 354
An epidemic may be restricted to one location, however if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic.[1]:55 The declaration of an epidemic usually requires a good understanding of a baseline rate of incidence; epidemics for certain diseases, such as influenza, are defined as reaching some defined increase in incidence above this baseline.[2] A few cases of a very rare disease may be classified as an epidemic, while many cases of a common disease (such as the common cold) would not."

ElaineBenes Fri 14-Sep-12 19:25:04

You can call it what you want. Definitions are meant to help, not hinder understanding. It's the concept which is important.

You asked why are there no epidemics? Well, there are. They're just small ones aka outbreaks.

ElaineBenes Fri 14-Sep-12 19:26:13

Tabitha

Outbreaks 'combining' would depend on how populations mixed with one another.

Tabitha8 Fri 14-Sep-12 19:27:43

Ah, ok. I'll change my question. The press coverage a year ago about measles implied a pandemic could be on its way. It never happened. I'm interested to know why.

ElaineBenes Fri 14-Sep-12 19:33:47

Enough people immunized their children to prevent it happening.

Tabitha8 Fri 14-Sep-12 19:41:32

So parents were vaccinating their previously older unvaccinated children? I've been looking for figures and have found this which talks about babies/toddlers.

www.guardian.co.uk/society/2011/jul/26/mmr-vaccine-take-up-rise
and it's from July 2011.

I might now have to post and run until tomorrow..... smile

minceorotherwise Fri 14-Sep-12 19:55:30

I was referring to the threshold, past which point the disease becomes stable
Has that been achieved?

LeBFG Fri 14-Sep-12 20:01:04

bm, where herd immunity is defined as the proportion of vaccinated people in a population then 3 in 1000 = 0.003 (or 0.3% as a percentage). Obviously, the effect this will have on disease incidence will probably be so small as to be immeasurable...but the point remains: the disease circulating in a reduced population. The three individuals are no longer sources of disease.

Wrt vaccines pushing disease into older groups and waning immunity, individuals with waned immunity are less ill when the contract the disease (thus spread it less etc). I read somewhere (though not sure how much I trust it) that in wc, individuals whose natually-acquired immunity has waned become more ill on reinfection than in individuals whose vaccine-acquired immunity has waned. It seems counter-intuitive, but who knows? I wouldn't mind seeing some papers of reinfection severity.

ginnybag Fri 14-Sep-12 20:07:10

I'm going back a few pages here but:

Will the people arguing about "rare" diseases please do some maths in the real world?

A minimum 25% fatality rate in a disease affecting 0.02% of people means, unprevented, 350,000 deaths, based on a UK population of 70,000,000.

70,000,000 x 0.02 x 0.25 = 350,000.

That's 350,000 deaths. Remember, that another 3/4 million have been sick and recovered.

I'm not suggesting that that would have happened in a flash - it would have distributed, but even so, that's a lot of additional strain on the NHS (and a lot of needless suffering) for something that's been prevented by a few drops on a sugar cube - and that's only one disease. Let's add back Diptheria, as well. Widespread Tetanus. Small pox.

All diseases that have been reduced down to near zero by forcing down the infection spread rate with artificial immunity - immunisation.

With the way we live now - all those planes, international holidays, shopping malls, huge universities in every major city, massive urban population centres, vaccination against everything possible is, frankly, essential. The idea that people are choosing to risk bringing these diseases back scares the hell out of me.

And I do think that they all, consciously or not, do so knowing that there's no real risk whilst everyone else vaccinates.

The sheer level of panic over the Swine flu thing was enough to convince me that, confronted with the reality of an 'uncontrolled' killer virus, most people are screaming for science to fix the problem!

Let's forget vaccination and think of it this way: 1 in 200 children are going to die. That's around about the still birth incidence, which I think everyone thinks is too high.

That's also the death rate for uncontrolled polio, based on the figures quoted earlier (25% of 0.02% infected. (200 * 0.02% = 4 * 25% = 1)

bumbleymummy Fri 14-Sep-12 21:21:07

No EB, it wasn't. You keep talking around it about decreasing susceptibility etc but you haven't actually answered the question. Shall I just assume that you are using herd immunity to mean the proportion of the population that are immune like the link I posted earlier so that you can say 'a little herd immunity'? If that's the case, what term do you use to describe the protection offered when a significant proportion of the population are immune?

seeker, I'm still struggling to see what my decisions wrt vacination have to do with this thread.

Math! It's a hypothetical question and it was directed at Elaine who seems to think that any immune people in the population means that there is some kind of protective effect, despite the fact that pretty much evey definition we've linked to says that a significant proportion of immune people is required for that to happen. Even your understanding of 'herd immunity' is different to EB's.

Jo, it is a basic one, yes. it came from a site I linked to earlier but also here Would you like to explain how you messed up your calculation? Maybe you need to brush up on your math and come back when you've got it straight wink

No mince, it hasn't.

LeBFG, I was asking EB if it counts as 'herd immunity' as in would it actually confer any protection to others.I'm pretty sure that we are in agreement that it won't and that it would not count as a 'significant proportion'. I would also like to see the reinfection info on whooping cough. It sounds interesting.

bumbleymummy Fri 14-Sep-12 21:22:04

Sorry math, not sure where the random (!) came from after your name. I'm not shouting at you or anything smile

bumbleymummy Fri 14-Sep-12 21:30:06

Ginny, 0.02% of those who contract polio.

Widespread tetanus?

Wrt flying bringing in lots of illness. There are plenty of diseases that are rife In other countries that we don't vaccinate against. Do you worry about them being brought in?

I wasn't panicking about swine flu. Tbh I think the ones who were panicking are the ones who are always looking for a science 'fix' to everything rather than actually looking at the disease itself and the likelihood of them actually getting sick and dying from it.

bumbleymummy Fri 14-Sep-12 21:40:42

Also Math, I'm pretty sure that no diseases have a threshold of 0.3%

ElaineBenes Fri 14-Sep-12 21:49:34

I think, BM, to have significant herd immunity, you would need a significant proportion of the population to be immune.

As I said above, given the exponential nature of disease transmission, I'm guessing that the function of herd immunity for a percent immune would be a S shaped sigmoid function. This means that the effect on herd immunity at low levels of immune in the population would be low and the increase on herd immunity of increasing the proportion immune would be disproportionately low. The effect in the middle of the curve would be greatest but the actual curve would vary according to the disease parameters even if the general shape of the relationship remains the same.

But I should say that I don't really know. I'm just looking at what would result from the epidemic theory model. If you know better, I'd be more than happy to stand corrected. It doesn't really matter since most routine vaccinations lead to significant levels of immunity within the population even if not sufficient to fully prevent outbreaks.

seeker Fri 14-Sep-12 23:26:02

"seeker, I'm still struggling to see what my decisions wrt vacination have to do with this thread."

Because if you express views that go against the currently accepted norms, it's useful to know where you're coming from.

And a person who would be happy for their child to spend the day at school sitting next to someone incubating polio has an unusual mindset or has knowledge the majority doesn't have. And it seems to me entirely reasonable to ask about that. And you seem very cagey about this.

Oh, and I wasn't bothered about swine flu either, because no one in my family fell into any of the vulnerable groups.

A minimum 25% fatality rate in a disease affecting 0.02% of people means, unprevented, 350,000 deaths, based on a UK population of 70,000,000.

70,000,000 x 0.02 x 0.25 = 350,000.

Um, I might be wrong but that doesn't look right to me.

Surely 0.02% of the population would be 0.0002 x 70,000,000? i.e. 14,000 so the death rate would 3,500.

Of course that is a lot of dead people which is awful but if you are looking at risk it makes a big difference.

I can't see where the 0.02% comes from though having scanned the thread back a few days. Was it the conversation on polio?

BM if it is the conversation about polio it is 0.02% of the whole population not 0.02% those who contract the disease.

ElaineBenes Sat 15-Sep-12 01:21:24

Good catch BBB. 350,000 people dying in an epidemic is Spanish flu epidemic proportions!

I agree with the sentiments of the post though - any preventable death is a tragedy.

You are right, EB, that any preventable death is a tradgedy so in a sense the figures don't matter. 3,500 is a lot of people in anybody's book so can be justfication enough to think about protecting yourself and your family against a disease.

However, I wouldn't want to see incorrect figures if it means that somebody might see it as an excuse not to face the reality of their decision to vaccinate because it is not that bad after all, or conversely, be frightened into a decision based on an incorrect view of the risk.

Not that this thread is about getting people to vaccinate - I think all minds are firmly made up.

ElaineBenes Sat 15-Sep-12 02:21:11

I couldn't agree more regarding good data and statistics.

mathanxiety Sat 15-Sep-12 04:12:39

'Also Math, I'm pretty sure that no diseases have a threshold of 0.3%'
They might have though. It depends on the way the disease is spread and how infectious it is and what population is most at risk. Hence my comments on different diseases having a different threshold.

'seeker, I'm still struggling to see what my decisions wrt vacination have to do with this thread'
To use the example of HIV example again, a decision not to use a condom ever while having sex in bathhouses with gay men would lead to what?
A decision not to vaccinate against Pertussis for instance could lead to infecting someone's baby, with horrible results. Effect of lack of herd immunity in Washington state, USA. Lack of herd immunity happens when PFBism runs amok and many parents decide individually not to vaccinate their little darlings.

'Wrt flying bringing in lots of illness. There are plenty of diseases that are rife In other countries that we don't vaccinate against. Do you worry about them being brought in?'
We have seen HIV hop continents. We have seen H1N1. Containing them is a priority.
We vaccinate against the flu strains that appear elsewhere and we track their progress, a new strain every year. We especially urge the elderly to get the annual flu vaccine.
We don't bother so much with a disease like malaria because we don't have the conditions that would cause widespread malaria here. But measures are taken in parts of the US to spray against mosquitoes and malaria used to kill people in the South. Now that the mosquito has been linked to West Nile Virus, increased attention is paid to the mosquito in North America.
Cholera is a worry even though the water supply is now safe. Cholera can be caught from undercooked seafood from contaminated water and other isolated individual foods that come in contact with a food handler or cultivator carrying the disease, but most cases in the west happen in people who travel without getting vaccinated properly. Cholera is always ready to cause an epidemic if conditions are right.
TB is a constant worry, especially now that there are bacteria resistant strains.

A brief history of brucellosis eradication in Ireland. Brucellosis remained an occupational hazard in Ireland for many in the beef and dairy business until 2009 when Ireland was declared brucellosis free by the EU, but there are still many left with the lingering effects of this disease. In the context of herd immunity (no pun intended) this is an interesting case and a interesting battle that illustrates the effect of a border with different policies on either side (which in turn illustrates the concept of herd immunity).

JoTheHot Sat 15-Sep-12 08:07:40

So bumble, you used a flawed formula to show my calculations were wrong. You agree with me that the formula is flawed (or basic to use your euphemism), and then ask again where I messed up my calculations. I realise this is likely just a stalling tactic.

Can you not see that if your working out was wrong (basic), then so to was your conclusion? You are insisting someone's sums are wrong even after having agreed that your calculator is defective. My calcualtions were 100%. You're grasp of mathematical epidemiology is inadequate.

seeker Sat 15-Sep-12 08:15:39

"'Wrt flying bringing in lots of illness. There are plenty of diseases that are rife In other countries that we don't vaccinate against. Do you worry about them being brought in?"

I'm sorry to bang on about polio, but this is obviously one of the reasons that it's important to vaccinate.

Smallpox has been eradicated, so there is no need to vaccinate against it any more. Polio is getting there, but not there yet.

bumbleymummy Sat 15-Sep-12 10:20:37

EB, to have herd immunity ( and here, I'm assuming that you are talking about the protective effect conferred on others) you need to have a significant proportion of the population vaccinated (as per the majority of the definitions posted here). If you don't have a significant proportion vaccinated then you don't have herd immunity ( see diagram I linked to earlier).

BBB, it's a form of polio, bulbar polio, which occurs in 0.02% of polio cases.

math, you don't need to point out that there are different thresholds to me. I posted about it earlier. A disease with a 0.3% threshold would disappear fairly quickly.

The whopping cough epidemic is due to waning immunity from the vaccine. There's a whole thread about it at the moment. Even the CDC recognised that it is not the unvaccinated driving it, the majority of cases are in vaccinated children.

Jo, the formula that I said was basic was for calculating HIT. You calculated R incorrectly. What formula did you use for that?

LeBFG Sat 15-Sep-12 12:10:19

The relevant question isn't 'are vaccinated people catching wc?'. The vast majority of people are vaccinated after all. The real question is 'are unvaccinated people catching it more than vaccinated?'. If they are, they are spreading it disproportionately.

JoTheHot Sat 15-Sep-12 12:14:07

I only recall my stating 2 values for R for illustrative purposes. Could you quote the bit where I calculated R?

bumbleymummy Sat 15-Sep-12 12:40:05

LeBFG, the majority of cases are in the vaccinated according to that link I posted on the other thread. The CDC have accepted that it is the waning immunity from the vaccine that is the cause of the outbreak. Do you disagree with them?

Jo,Do you not think if you are doing something for illustrative purposes that your figures should be correct? Particularly when you go on to say "This is very clear and totally irrefutable'.

The formula isn't 'flawed' btw but it is basic/simple. You said earlier:

"You use this formula to question my calculations. Unfortunately, this formula gives the wrong herd immunity threshold for every single disease in this table."

The formula actually gives the correct threshold for every figure in that table.

For example:

Diptheria R0 = 6-7 (Let's assume 7) Using the formula HIT = 1 -(1/R0)
HIT = 1 - (1/7) HIT = 1 - 0.14 HIT = 0.86 or 86%. If you use R0 = 6 you get 83%. The given threshold in the table you linked to is 85% - the average of the two.

It works for the other ones in the table as well. Why did you think they were wrong? Were you using the formula incorrectly? It is a basic/simple formula. I'm not sure how you went wrong with it.

LeBFG Sat 15-Sep-12 13:14:08

I'm glad you changed epidemic to outbreak, bm.

Of course the majority of cases are vaccinated, most people are vaccinated after all. To illustrate: more babies die of SIDS when they sleep in the parental bed than on sofas...because that's what most people now do. There is still an a higher proportion of deaths on sofas because this is riskier. What about the proportion of wc in unvaccinated people?

I'm struggling to keep up with these two threads. Please repost the CDC link saying the outbreak (Washington is what we're referring to I think) is caused by waning vaccines so I can read and comment. Thanks.

bumbleymummy Sat 15-Sep-12 14:01:10

BFG, there was epidemic declared in Washington state (and Colorado too) but the article I am referring to (not a CDC link) is talking about the outbreak of whooping cough in the US in general.

The other thread isn't too big. I'm sure you can find the information yourself.

LeBFG Sat 15-Sep-12 14:07:48

Cheers

LeBFG Sat 15-Sep-12 14:18:06

As I thought, math's link refers to Washington and your statement is about the whole of the States. I've found this - is that the correct one?

JoTheHot Sat 15-Sep-12 15:08:34

An epidemiological model for R(0) minimally needs terms for host density and disease transmission rate. Your formula has no term for disease transmission, and so is flawed. It also ignores the fact that populations are spatially structured and a whole host of other stuff.

Disease transmission is usually dependent on host density, whereas your formula in effect assumes transmission rate is independent of host density, i.e. it ignores the fact that when people are in closer proximity, a disease will pass more easily between people.

I don't see how illustrative figures can be correct or incorrect. They should be illustrative and plausible. Your argument is basically as follows. I said 'consider a case where oranges cost 10p, there's a drought and the price goes up 12p'. Rather than just accept the point that oranges are more expensive after a drought; something which you had previously been denying. You find the simplest economic model known to mankind, which shows that if there was a drought, orange prices would increase 10%. On the basis of this, you say repeatably that my figures are wrong. This allows you to avoid the clear and irrefutable point that vaccinating even a tiny number of people always reduces the rate of spread of a disease. To this extent, people who vaccinate benefit people who don't.

bumbleymummy Sat 15-Sep-12 19:46:18

Yes, BFG, that was the one I was talking about. Are you suggesting that waning vaccine immunity didn't have anything to do with the epidemic in Washington?

From Maths link:

"Making things worse, it seems, is an increase in cases among children aged 13-14. Children get a booster shot at age 11-12, but the new outbreak indicates that the effectiveness of the booster may not last very long. The dramatic increase in whooping cough this year also suggests that the bacterium that causes it, Bordetella pertussis, is mutating to make the vaccine less effective. "

It seems to be the author's own opinion that the anti-vaxers are to blame.

Jo, the formula you called 'flawed' is the HIT formula not the formula for R0. You really do seem to have gotten lost here. How do you explain the HIT formula being flawed when it actually calculates the thresholds correctly? (As shown above).

You illustrative figures were incorrect because if you have 10% immunity in the population then R would not be equal to 2.9 as you stated, it would be equal to 2.7, as I've already pointed out. I know you don't want to admit that you made a silly mistake but really?

mathanxiety Sat 15-Sep-12 20:17:45

Herd immunity is not quite 'the protective effect conferred on others'. It is the threshold which once reached means a disease is unlikely to spread. It is an epidemiological concept; the term does not imply that unvaccinated individuals or those with weakened immunity will not catch the disease. If you are not vaccinated you could still catch the disease because of your lifestyle, if the individuals you come in contact with are also unvaccinated and infected, because of the level of infectiousness of the disease, or because of the particular segment of the population a disease tends to strike..

Wrt the allegation that it is waning vaccination effectiveness that causes the rise in whooping cough cases -- this is amply disproved by the fact that there are pockets of vaccine resistance in the US and these pockets tend to be the places where WC is at its highest levels. Washington, Wisconsin and Montana have the highest rates of vax refusal and WC epidemics. For those cases where waning immunity gives rise to an upsirge in WC, lack of vaccination is also the cause -- there is a schedule of boosters that everyone is supposed to get, beginning at 11-12 iirc.

Scroll down to table 3 showing exemptions granted to vax requirements by state for religious or philosophical reasons and note the percentage column, third from the right.

States with highest Pertussis rates in the US. It is interesting to note that for those states with a high rate of pertussis that are not also high vax refusers, all are contiguous to states with rates of vax refusal over 1%. In addition, some have a border with Mexico. Some have a high number of immigrants and migrant workers.

Adults need to get a booster because they are not being reexposed frequently as they used to be when diseases were widespread in their wild form. Either you get exposed naturally or you get exposed artificially via vaccine. If you can't get the natural exposure then you should seek out the artificial exposure.

'Historical Comparisons of Morbidity and Mortality for Vaccine-Preventable Diseases in the United States' -- comprehensive overview of the effect of vaccination policy in the US.

ElaineBenes Sun 16-Sep-12 00:42:21

I think that's an important point math. It's a probabilistic model so there are no guarantees. It also assumes the population mixes randomly which, as you point out, isn't the case - anti vaxers tend to hang out with one another!

seeker Sun 16-Sep-12 07:07:14

Could somebody explain to me in simple trms for the hard of thinking why we're spending so much time on defining our terms?

I thought "herd immunity" meant that enough people within a community were immune to q particular disease to make it extremely unlikely that a non immune person entering that community would contract whatever it is.

LeBFG Sun 16-Sep-12 07:25:58

Finally, bm, thanks for being so helpful. I'll return the favour sometime.

I think the CDC comment you made came from page 3:
While some parents around the country have taken a stand against childhood vaccines, the outbreak is not being driven by unvaccinated children, according to the CDC. Most of the illnesses are in vaccinated youngsters, officials said.

From this statement it isn't clear what the 'officials' said and what the CDC said. It appears the statements are linked (i.e. the outbreak is not being driven by unvaccinated children because we see most of the illnesses...in vaccinated youngsters), but in fact they aren't. I would love to see an original CDC statement wrt to this point.

Evidence for waning immunity, bm, comes from the resurgence of wc in the age group 13-14 who exclusively had the new vaccine, not purely from the the fact vaccinated people catch wc (they always have done).

From saintly's CDC link:
Given the high transmissibility of B. pertussis, a proportion of vaccinated persons remains susceptible and can become infected during a pertussis outbreak. Unvaccinated children have at least an eightfold greater risk for pertussis than children fully vaccinated with DTaP (7). However, because in most of the cases the patients were vaccinated, the 4.5% of Washington school children who were exempted from vaccination during 2011–2012 represented only a small proportion of those at risk for pertussis in the state. Although vaccinated children can develop pertussis, they are less infectious, have milder symptoms and shorter illness duration, and are at reduced risk for severe outcomes, including hospitalization (8–10).

The ongoing pertussis epidemic in Washington reflects the evolving epidemiology of pertussis in the United States. Although acellular pertussis vaccines provide excellent short-term protection, early waning of immunity might be contributing to increasing population-level susceptibility

Note the use of healthy scientific skepticism I've highlighted in the text bm.

All-in-all, your bald statement:
The whopping cough epidemic is due to waning immunity from the vaccine... Even the CDC recognised that it is not the unvaccinated driving it, the majority of cases are in vaccinated children. is unsubstantiated.

LeBFG Sun 16-Sep-12 07:48:19

Seeker: I know how you feel. There is, however, a logic behind this dispute and it all lies in the definition of three terms: herd immunity, herd immunity effect and herd immunity threshold.

People use herd immunity inappropriately for ease or through misunderstanding. It simply means 'proportion of people vaccinated'

When a particular proportion of people are vaccinated (the threshold value) the disease should die out rather than propogate in a population thus effecting some level of protection to the unimmune. It is this magic number governements/health authorities are always trying to hit.

Bm denies there is a protective effect (herd immunity effect) to the unimmune below threshold values. e.g. if we vaccinate at levels 5% less than threshold the protective effect to unimmune is zero. We think this is because she wants to assert that in diseases where we've not reached threshold, vaccinating does not protect, to any extent, the unimmune.

seeker Sun 16-Sep-12 07:51:35

Thank you. I understand now. I was feeling blinded by anti-science.

minceorotherwise Sun 16-Sep-12 07:58:50

Oh? I thought herd immunity referred to the phenomena of what happens to a disease when we reach that number
Not an arbitrary term for levels of protection under that number?
But I guess it's just semantics isn't it?

bumbleymummy Sun 16-Sep-12 08:27:30

Math and LeBFG:

""The substantial majority of the cases are explained by this waning immunity," said Dr. William Schaffner, an infectious-disease specialist at Vanderbilt University.

You can argue with Dr Schaffner if you don't believe it.

It's fairly obvious that if immunity from the vaccine wanes then you are left with more susceptibles in the population so the disease will continue to spread. That is the problem they are encountering at the moment and is the reason they are recommending more boosters.

Yes, most people will use the term 'herd immunity' to mean that and many links posted on the thread support that usage although it can also be used to mean the proportion of a population who are immune which is where the confusion has come in. While it is possible to argue that 'a little bit of the population are immune' it is not possible to argue that you can have 'a little bit' of herd immunity in the other usage because, as many links and diagrams have pointed out, a significant proportion of the population need to be immune for that to exist.

bumbleymummy Sun 16-Sep-12 08:33:43

Math and LeBFG:

""The substantial majority of the cases are explained by this waning immunity," said Dr. William Schaffner, an infectious-disease specialist at Vanderbilt University.

You can argue with Dr Schaffner if you don't believe it.

It's fairly obvious that if immunity from the vaccine wanes then you are left with more susceptibles in the population so the disease will continue to spread. That is the problem they are encountering at the moment and is the reason they are recommending more boosters.

Yes, most people will use the term 'herd immunity' to mean that and many links posted on the thread support that usage although it can also be used to mean the proportion of a population who are immune which is where the confusion has come in. While it is possible to argue that 'a little bit of the population are immune' it is not possible to argue that you can have 'a little bit' of herd immunity with the other usage because, as many links and diagrams have pointed out, a significant proportion of the population need to be immune for that to exist.

bumbleymummy Sun 16-Sep-12 08:34:16

Sorry for double post - phone issue!

bumbleymummy Sun 16-Sep-12 08:36:08

I have asked several times on the thread for people to clarify what they are talking about but they still keep jumping between the definitions so I'm not surprised that people are confused.

JoTheHot Sun 16-Sep-12 08:37:55

bm, epidemiological models are not like formulas in geometry or mechanics. They do not produce answers which are right or wrong. They produce estimates which are good or bad. The model you are using to claim my illustrative figure is wrong, is like a road map which only shows the motorway network. I said when I drove from Cambridge to Norwich I saw a red kite. You keep claiming I made a silly mistake because there's no such road. I've tried to persuade you that there is, but you won't have it. So let's get back to the point of the illustraion, the red kite:

Do you now accept that vaccinating any proportion of people, no matter how small, will reduce the incidence of that disease in the unvaccinated proportion of people? Or do you still believe that there is a cliff-edge effect, such that the reproductive rate of a disease doesn't influence the incidence of the disease unless it falls to one or below?

LeBFG Sun 16-Sep-12 09:04:19

Yes it is semantics minceorotherwise.

I think the confusion arises over herd immunity is many people, scientists included, are talking about threshold effects in the context of vaccination. They'll talk about attaining herd immunity to mean vaccinating to arrive at a level where theoretically the whole herd becomes immune to a disease. In reality, this doesn't happen, threshold just describes the progression of the disease and a population vaccinated to 99% will still have cases of the disease. The unimmune aren't totally excluded from contracting the disease (as pointed out earlier). In this debate we are particularly arguing about protective effects from vaccination ^below threshold level^: bm asserts there are none. The literature says otherwise. In this context, using herd immunity to mean 'proportion of vaccinated/immune people' is the correct usage.

I have no problem with Dr Schaffner's opinion on the subject, bm. I object when you explicitly say 'caused by' thereby implying sole reason, whereas the Dr says 'substantial majority' implying important reason, with other causes. Semantics again.

bruffin Sun 16-Sep-12 09:16:50

Think about smallpox in New Zealand and Australian. Smallpox was not in the community only through people arriving in ports and harbours. They vaccinated the people they came in contact with and a people within a certain radius, but did not vaccinate the majority of the population, only a small proportion. Antivaxxers including BM have argued that smallpox could not have been irraditicated by vaccine in these areas because of the low vaccination rates, but that is because they do not understanding how herd immunity works differently for different diseases and communities.

bumbleymummy Sun 16-Sep-12 09:17:08

Jo, are you trying to say that, in your example, there is a way that R could have equalled your 2.9? Really? Why can't you just admit you made a silly mistake? I'm also interested to know how you managed to use the HIT formula incorrectly and come to the conclusion that it didn't calculate any of thresholds in your link correctly when it did. Btw "I'm finding it not inconsiderably entertaining to find you demonstrably and unambiguously flailing around" wink

bruffin Sun 16-Sep-12 09:29:09

Why don't you understand that Jo didn't calculate 2.9, she used it for illustrative purposes nothing more.You are arguing to cover your inadequacies, just as you have done on the other threads. You made a huge mistake on this thread with the graph and made out it was nothing. It illustrated how little you really do understand because if you understood the subject you would have realised that the graph could not possible have represented what you were saying it did.
Elaine's "Black Knight " is a perfect description of you.

bumbleymummy Sun 16-Sep-12 09:29:27

"They'll talk about attaining herd immunity to mean vaccinating to arrive at a level where theoretically the whole herd becomes immune to a disease."

Not quite BfG, they'll talk about reaching the threshold so that the disease will become stable in the community or exceeding it so that it will die out.

I also haven't said 'there are none'. Why don't you read what I'm actually writing?

Bruffin, a) I'm not anti-vax and b) I wouldn't argue that because what you are talking about is a different method of containment.

JoTheHot Sun 16-Sep-12 09:31:28

R could not have equalled 2.9. I made a silly mistake. I used the HIT formula wrong because I'm congenitally innumerate. There is no road between Norwich and Cambridge.

Will you now answer the question?

Do you now accept that vaccinating any proportion of people, no matter how small, will reduce the incidence of that disease in the unvaccinated proportion of people? Or do you still believe that there is a cliff-edge effect, such that the reproductive rate of a disease doesn't influence the incidence of the disease unless it falls to one or below?

bumbleymummy Sun 16-Sep-12 09:34:01

Lol . I've actually been thinking of Jo as the black knight grin

bumbleymummy Sun 16-Sep-12 09:45:59

Thanks Jo smile

I don't think it's a 'cliff edge effect'. If you read my posts and my links you'll understand what I'm saying. Do you think that having 3 people in 1000 (a very small proportion) immune will offer any protection to the non-immune? There is a reason why most of these definitions use terms such as 'a significant proportion' you know.

seeker Sun 16-Sep-12 10:06:08

Well, I am an arts graduate who counts on her fingers. But.

It would seem to me to be logical that any level of immunity within a population must by definition reduce the risk of transmission of an infectious illness a bit- simply because that's one less person who might pass it on. So in a group of 10 people, where 9 are immune, the 1 remaining non immune person is not going to catch disease Z from them. If 5 are immune and 5 aren't, the chances are higher, but still lower than if none of the group is immune. So even 1 immune person in the group is going to reduce the probability of catching Z a bit.

Reducing the risk of catching Z is different from eradicating it. Which would mean,in my example, 9 of them being immune so that Z has nowhere to go after it has infected and either killed or conferred immunity on the one non immune person.

Does this make sense, or have I missed something?

JoTheHot Sun 16-Sep-12 13:23:35

You're explanation is excellent seeker. Bm is still being coy as to whether or not she fully accepts it.

Here is an online simulator you can play with. It shows that increasing the proportion of people who are initially immune, always reduces the proportion of the unimmune people infected in a disease outbreak. The effect of 0.3% immunity is too small too see, but 5% has a visible effect. If you'd like me to talk you through how to use it bm, just ask.

Tabitha8 Sun 16-Sep-12 18:54:39

You could try explaining it to me. I can't even count on my fingers.

seeker Sun 16-Sep-12 18:59:31

Well, I understood myself, Tabitha- don't you understand me?

< image of self as cutter through of jargon collapses about ears>

Tabitha8 Sun 16-Sep-12 19:06:51

I can follow the 1 person in a group of ten immunity example, but we, in reality, mix with far more than that. They then mix with others.
Now, where am I? I think I'm lost already. grin

seeker Sun 16-Sep-12 19:41:59

In my head it just works the same with bigger numbers. Imagine a class at school, or a nursery, and imagine 3 out of 30 instead of 1 out of 10. Or 10 out of 100.

mathanxiety Sun 16-Sep-12 21:28:00

BM, 'waning immunity' means no boosters are being administered. Therefore lack of vaccination remains the issue. The Vanderbilt doctor is only partially right, or you have posted a partial quote. Or you have misparaphrased him.

'While it is possible to argue that 'a little bit of the population are immune' it is not possible to argue that you can have 'a little bit' of herd immunity with the other usage because, as many links and diagrams have pointed out, a significant proportion of the population need to be immune for that to exist. '

Depends on the disease and on the part of the population most susceptible. (See Briffin's post about smallpox in NZ and Aus)

mathanxiety Sun 16-Sep-12 21:33:03

BM -- 'I don't think it's a 'cliff edge effect'. If you read my posts and my links you'll understand what I'm saying. Do you think that having 3 people in 1000 (a very small proportion) immune will offer any protection to the non-immune? There is a reason why most of these definitions use terms such as 'a significant proportion' you know.'

Again, you are missing the point about different diseases, different means of spreading them, different susceptibility levels in different parts of the community.

Hospital workers and Hepatitis of various kinds is a case in point.

LeBFG Mon 17-Sep-12 07:31:20

I think this point about 'significant proportion' needs addressing. I would understand this to mean: a proportion that would have a significant/measurable effect of disease susceptibility in the unimmune. If it were explicitly referring to threshold, they would be emphasising the exact proportion etc.

Bm: explain to us explicitly what your understanding of herd effect is. Do you think that vaccinating 3 in 1000 will have an effect? 400 in 1000, 800 in a 1000?

Tabitha, you ask a good question wrt sources of disease. We brush shoulders with perhaps tens, of people a day (more if you're walking through a shopping mall etc). Still, most of the diseases we vaccinate, although often pretty contagious, are caught laregely from people we have close contact with, very frequently family members. Children have always been very susceptible to disease bacause, amongst other things, they are in close contact with a lot more sources of disease at nursery/school than we are as adults. I reckon my weekly close contacts come to less than ten people (but I live in the country) and even when I was working, you could have added 5 to that number. You may be surprised by how few close contacts people have in a week! Factor in a low vaccination rate - it is quite possible that having one or two of my contacts no longer spreading disease could have a big impact on if I get ill or not.

bruffin Mon 17-Sep-12 08:11:56

I do think there is a difference if you commute somewhere like London LeBFG, you can come into very close contact with a lot of different people on a daily basis
But if you dont have children, drive to work and work in a small office then you really don't need to a lot of people around you to be immune to stop you getting a disease. I know before we had children, dh started to commute for a short while and started picking up one bug after another, which he never did before.

LeBFG Mon 17-Sep-12 08:28:11

Yes indeed bruffin. Lifestyle will make a big difference to your exposure risk. The thing with epidemiological models is they deal in populations writ large, not micro-populations, or families or even individuals. It may seem improbable that a low vaccination rate will have any effect on whether you contract a disease or not, but where an individual isn't seeing too many people and the disease is not super contagious, I can easily imagine a low proportion of vaccinated people making a difference on disease incidence.

bruffin Mon 17-Sep-12 09:30:15

It just plain common sense, not sure why we need huge long threads on it.

Tabitha8 Mon 17-Sep-12 19:00:01

Bruffin It was a health visitor who said to me that I should vaccinate my child in order to maintain herd immunity.
Then we hear the term "herd immunity" in the media whenever there is an outbreak of disease.
Here, on the BBC website, for example, during last May's measles cases, they talk about an uptake rate for the MMR required in order to stop the spread of disease:
www.bbc.co.uk/news/health-13561766
"HPA figures show that the number of toddlers getting the MMR vaccination is climbing steadily, but is still far from the 95% uptake rate needed to stop the spread of the disease in the community."

I think that this thread might, more sensibly, be suggesting that the above approach is too simple? It will vary from area to area and, as BFG said, from circumstance to circumstance?
Yet, there again, I do go on holiday. I travel abroad, I travel within the UK.

ElaineBenes Mon 17-Sep-12 21:28:05

But as we've discussed, while there is indeed a threshold where a disease can't be sustained - even below that threshold, having more immunised children still reduces the probability of a non immune child being exposed to the disease.

Primarily you should vaccinate your child to protect him or her but even if you're below the threshold where a disease won't spread, you can maintain the herd immunity already achieved by keeping vaccination levels up. That must be what your hv meant.

bruffin Mon 17-Sep-12 22:09:36

"HPA figures show that the number of toddlers getting the MMR vaccination is climbing steadily, but is still far from the 95% uptake rate needed to stop the spread of the disease in the community."

Most people reading that would have the common sense to realise that 95% is not an all or nothing figure.

ElaineBenes Mon 17-Sep-12 23:01:51

Not if you want to pretend that herd immunity is a myth so you can keep on claiming that your decision not to vaccinate does not impact anyone else in society.

sashh Tue 18-Sep-12 02:32:05

Elaine

That still does not explain how R>1 can rise to R=1

ElaineBenes Tue 18-Sep-12 11:32:26

R decreases as the number of susceptible people in the population decreases. It doesn't necessarily stop when it equals 1.

Obviously it doesnt rise if you're going from more than 1 to less than 1. I think bm wrote that by mistake?

Tabitha8 Sat 22-Sep-12 21:37:24

Bruffin "Most people reading that would have the common sense to realise that 95% is not an all or nothing figure. "
That's one person here, then, with no common sense, I am afraid.

ElaineBenes Sat 22-Sep-12 23:41:54

Tabutha
Are you still saying that there is no effect on disease transmission if vaccination rates are 90% as opposed to 95%?

Even bm has realised that there is an effect.

LMCG Tue 25-Sep-12 09:11:47

If people are so confident that vaccination works, why are you worried about whether or not other people are vaccinated??? surely you should be protected by said vaccination? herd immunity is shown time and time again to be a complete myth - outbreaks of disease occur in fully vaccinated communities.

ElaineBenes Tue 25-Sep-12 15:08:54

Oh dear, someone's been drinking the crankosphere kool aid.

Have you actually read this thread and the links on it?

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