Herd immunity: how vaccines protect the unvaccinated

(Image credit: Screenshot/YouTube)

last updated 8 January 2015

Vaccines are a fundamental building block of modern public health, and everyone who can should get them. But some people can't be vaccinated — very young babies, the elderly, or people who have allergies, depending on the particular vaccine. However, a rigorous vaccination program can still protect those people.

How? As Aaron Carroll explains below, it's all about the statistics of disease. In order for an outbreak to spread, there must be a transmission mechanism — typically, another person who catches the disease and then gives it to someone else. But if there are enough people who are immune to the disease surrounding the sick person, then the outbreak can't get going.

For example, chicken pox used to kill a few children each year. But after the chicken pox vaccine became ubiquitous, the death rate for babies under one year of age plummeted, all the way to zero between 2004-2007 — and kids that young aren't even old enough to receive the vaccine! Ubiquitous vaccination made it so those babies weren't exposed to the disease, and thus saved many lives. Check out the full explanation in the video below. --Ryan Cooper

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Ryan Cooper is a national correspondent at TheWeek.com. His work has appeared in the Washington Monthly, The New Republic, and the Washington Post.

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