Obviously for a disease to spread, it has to go from one person to another (either directly or indirectly). A man with measles on a deserted island won’t spread it to anyone because there is no one else who can catch it. Same thing with an infected man in the midst of a group of people immune to the disease.
If we start adding people to the island, then it can spread. And if we put the man with the measles into a group of susceptible people, then it will flare up into a bona fide epidemic (think America when the first Europeans landed).
Now imagine a city where most of the people are immune. If the man with the measles goes there, odds are he won’t meet anyone susceptible. When he gets better, the disease will disappear. This is herd immunity.
Basically the immune people shield the susceptible ones from the disease. They act as sort of a fire break that keeps the epidemic from spreading. Perfectly reasonable idea.
Of course, it is possible that even though this makes sense it doesn’t actually happen in the real world. I decided to look back at the literature and see if there are any real examples of herd immunity out there. There are lots of them. Here are two.
More than a hundred years ago, scientists were noting that not everyone had to be vaccinated against smallpox to stop an epidemic in its tracks*. Scientists also noticed that when they were wiping out smallpox, not everyone needed to be vaccinated to have the disease disappear. In many places, it was enough if 80% of the population could be vaccinated and revaccinated in a 4-5 year period. So smallpox definitely provides evidence for herd immunity.
A common argument against statistics like these is that the decrease in disease came not from vaccines but from better hygiene. To try to counter this argument, I decided to look at mumps.
A mumps vaccine wasn’t really widely available in the U.S. until 1968. Here is a quote from a really thorough review article from 1993 in the journal Epidemiological Review :
"Mumps notifications have now fallen by more than 95 percent since the introduction of vaccination. Given that vaccine uptake has only recently reached that level among school entrants, that uptake among preschoolers is far below that level, and that mumps vaccine efficacy is probably below 90 percent, this decline in incidence is appreciably greater than would be predicted by direct protection alone. Assuming that the decline in reported cases reflects incidence and not a decline in notification efficiency, then this is evidence for indirect protection of susceptibles by herd immunity."
Since mumps vaccinations didn’t start until 1968, it is pretty unlikely that its decrease has been due to better sewers and washing our hands more thoroughly. No, this is a direct result of vaccines and herd immunity.
In fact the whooping cough epidemic can be explained by herd immunity too. It isn’t that not enough people are vaccinated (at least not yet). Instead, it is that the vaccine has worn off for older kids increasing the number of susceptible people. We now have too few people to have a solid fire break and so flare ups of whooping cough are starting to happen.
So despite what you might read on the web (including in the comments section of this blog), herd immunity is real. There is an overwhelming amount of data out there to support the idea that it protects us from the diseases that used to sweep through our population. Herd immunity is a fact and therefore real whether you believe in it or not.
*Farr W. Second annual report of the Registrar-General of Births, Deaths and Marriages of England and Wales, 1840.
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High numbers of vaccinated people act like a fire break