Herd immunity

The top box shows an outbreak in a community in which a few people are infected (shown in red) and the rest are healthy but unimmunized (shown in blue); the illness spreads freely through the population. The middle box shows a population where a small number have been immunized (shown in yellow); those not immunized become infected while those immunized do not. In the bottom box, a large proportion of the population have been immunized; this prevents the illness from spreading significantly, including to unimmunized people. In the first two examples, most healthy unimmunized people become infected, whereas in the bottom example only one fourth of the healthy unimmunized people become infected.

Herd immunity (also called herd effect, community immunity, population immunity, or social immunity) is a form of protection from infectious disease. This happens when a large percentage of a population has become immune to an infection, whether through previous infections or vaccination.

When many individuals have immunity, they do not spread the disease any more. This either stops or slows the spread of disease.

While not every single individual may be immune, the group as a whole has protection. This is because there are fewer high-risk people overall. The infection rates drop, and the disease peters out.

Herd immunity protects at-risk populations. These include babies and those whose immune systems are weak and can’t get resistance on their own.