Flu week: herd immunity

Vaccinations are one of the wonders of the modern world.  Most people think primarily of the first order benefits- I got a vaccination, therefor I don’t get sick.  Hurray!  Then there are the second order benefits- I got a vaccination, therefor I don’t get sick, therefor I can’t infect anyone else.  Hurray!    If enough people are vaccinated, this culminates in  a third order effect, herd immunity.

Most diseases are only infectious for a short period of time.  If they don’t infect at least one other person while you’re contagious, that line dies out.*  If a disease’s average transmission rate falls below one because there are so few infectable individuals, it will be impossible for the disease to sustain itself.  This is called herd immunity, and it means even vulnerable people (such as those with compromised immune systems) are safe.  If the population is “the entire world”, you can eradicate a disease entirely  This is what we did with smallpox.

This is why refusing to vaccinate your child didn’t have many consequences for the early adopters.  As long as vaccination levels in your local population (e.g. school) were high enough to provide herd immunity, and you didn’t travel to an area where the disease was endemic, you were safe.  But when enough people in the same population forego vaccines, the disease is able to get a foothold, with disastrous consequences.

The idea that vaccines cause autism is conclusively disproven, but I don’t find it unimaginable that some vaccines have some subtle negative side effects, or rarely trigger massive negative reactions  in unpredictable individuals.  But the cost of the vaccine only exceeds the cost of not getting the vaccine when everyone else is getting vaccinated.  If you’re in a population without herd immunity, the MMR vaccine is waaaay better for you than measles or mumps.**

This concept can extend beyond vaccinations to anything that lowers transmission rate, including hand washing, face masks, condoms, and HIV pre-exposure prophylaxis.  This why “what makes sense for an individual?” and “what is best for the population?” often have very different answers.


*You might be asking “But won’t they run out of people eventually?  How does any disease persist?”.  That is the subject of my undergraduate thesis, and also decades of work by hundreds if not thousands of brilliant people.  We have only a fraction of the answer.

**Rubella is actually not very dangerous at all, except for being absolutely devastating to the fetus if the mother catches it in a certain stage of pregnancy.  We vaccinate for it to protect those fetuses, not the vaccine recipients.