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?

'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.

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

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.

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?

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.

asked and answered way way back in the thread bm

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.

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?

'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.

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?

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?

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!

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

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?

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 (επί (epi)- meaning "upon or above" and δήμος (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."

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.

Tabitha

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

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.

Enough people immunized their children to prevent it happening.

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.....

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

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.

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)

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

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.

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