Herd immunity is the idea that if enough people get immunized against a disease, they’ll create protection for even those who aren’t vaccinated. This is important to protect those who can’t get vaccinated, like immunocompromised children.
You can see in the image how low levels of vaccination lead to everyone getting infected. Medium levels slow down the progression of the illness, but they don’t offer robust protection to the unvaccinated. But once you read a high enough level of vaccination, the disease gets effectively road-blocked. It can’t spread fast enough because it encounters too many vaccinated individuals, and so the majority of the population (even the unvaccinated people) are protected.
Find out more here.
So, a couple of things…
I’ve heard some irresponsible parents be like “Eh…. I don’t have to, there’s this thing called herd immunity that will protect *my* kid if I don’t vaccinate them.” This graphic is super useful in showing that (when it comes to measles) even small dips in vaccination rates can leave a child unprotected.
And second… some kids can’t get vaccines. They have complex health problems, or they have legitimate reactions to vaccinations and have to have their vaccinations spread out. And some kids are unintentionally left unprotected for a little while, maybe they missed doses because of a chaotic home life, or fluctuation in health care coverage. These people are a small part of the population, but that alone probably hovers at 2-3%… so small changes in vaccine rates can put a whole “herd” below that 95% coverage pretty easily.
That’s for measles of course, other diseases are usually less infectious, but there’s a tipping point for every disease (known as the R0).
Also, some people who are vaccinated, still can get the disease (known as breakthrough disease). This is common in influenza & chicken pox… however those vaccinated individuals are usually less likely to infect others than unvaccinated individuals who have the disease.
A Middleground 