Herd Immunity

A very quick post as I’ve loads of other things I should be doing. Like ironing shirts for the first day back tomorrow. But I got into a brief discussion on Twitter about measles data being a topical way to get kids thinking about patterns, and it was pointd out to me that I never finished off my mini-scheme about MMR from ages ago. Obviously this is topical (and tragic) at the moment. I don’t have time to do this properly, but this is the easiest way to share the ‘herd immunity’ activity I put together then.

Herd immunity is a simple idea; if most of the population are protected against a disease, then it is much less likely to spread and so even those unvaccinated are effectively protected. Scientific views about the precise percentage that need to be immune vary, but it’s certainly well above the current proportions, especially for MMR. This can be blamed on the media, who gave headlines and airtime to Wakefield (struck off for poor ethics, not to mention falsifying data) and many of the anti-vaccination groups. Some organisations, like the BBC, say they need to provide ‘balance’. Others know better but seem to like the increased sales the scare stories bring in.

Anyway, enough ranting. Back to herd immunity.

Some people aren’t vaccinated yet or their vaccinations, for many reasons, have resulted in them being less than 100% protected. This group includes young babies, the immune-suppressed and so on. This is less than ideal but unavoidable. Others remain unprotected because they or their parents believed the media intead of the science. There’s some interesting research to show that people feel more guilt when they have acted to cause a problem, than when their inaction causes an equivalent problem. I just can’t find the link – anyone?

If you never meet anyone who can pass the pathogens on to you, then you won’t get the disease. It’s simple. So more people protected means less chance of bumping into someone who gives you horrible germs. Yes, I’m simplifying, partly because I’m a physicist and partly to put this in classroom-suitable terms. This is herd immunity, where the whole population is effectively protected because enough of them are actually protected. (It has the additional benefit that by removing possible reservoirs there’s less mutation and outbreaks tend to be less severe.)

Herd Immunity as a pdf

The above worksheet gives students a chance to see why even those who are unvaccinated get protection. You’ll need to give them the background, or send them off to research it. Hopefully not just on Wikipedia.

Finally, some extension questions:

  • should immunisation be compulsory when medically possible?
  • should vaccination be required before starting school (true in some parts of the USA)?
  • is it unethical to rely on herd immunity if you are not prepared to risk the small but measurable (millions to one) possibility of adverse effects of a vaccination? (I mentioned this in a guest post for NoodleMaz in January)
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2 thoughts on “Herd Immunity”

  1. “Herd” immunity is a vital element in immunisation strategies as it’s what protects those who are, for whatever, unable to be vaccinated against infectious diseases. This is a concept that so many of the anti-vaxxers fail to understand – but scientific understanding is not their strong point. At the recent QED meeting in Manchester, Dr. Rachael Dunlop used the term “community immunity”. I think this is a great term, as it conveys the message that sufficiently widespread vaccination not only protects the vaccinated individual but also imparts protection on other, possibly more vulnerable, members of one’s community. It gives the message that individual actions have consequences for one’s friends, families and neighbours, whereas “herd immunity” may be misinterpreted as implying that people are likes herds of animals, which some may misconstrue. Perhaps public health doctors and scientists should review the language they use to describe the benefits of vaccination?

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