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?

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.

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?

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

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.

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.

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

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.

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. "

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.^

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

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.

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.

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 ...

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!?

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!'

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.

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.

"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.

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?

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.

"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?

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?

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.