Herd Immunity

[Tabitha8]

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?

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.

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

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.

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.

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

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

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?

*Elaine, in order for a disease to become stablein the community, R must be 1 rise to R=1

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.

'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.

'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.

"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.

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.

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.

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.

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

that was supposed to be a

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.

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.

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.

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).

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.

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

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.

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?