When most of the people in a community are immunized, there is less opportunity for a disease to take hold in that location and make people sick. This is known as “herd immunity” or “community immunity.” Because there are members of our society who can’t receive vaccines for medical reasons or because they’re too young, they rely on herd immunity to keep disease out of their homes, schools, and neighborhoods. On a smaller scale, herd immunity affects families, too. If infants are immunized, they’re less likely to contract diseases in a child-care setting, which can be potentially dangerous to other relatives such as elderly grandparents, and vice versa.
When a critical portion of a community is immunized against a contagious disease, most members of the community are protected against that disease because there is little opportunity for an outbreak. Even those who are not eligible for certain vaccines—such as infants, pregnant women, or immunocompromised individuals—get some protection because the spread of infection is contained. This is known as “community immunity.”
In the illustration below, the top box depicts a community in which no one is immunized and an outbreak occurs. In the middle box, some of the population is immunized but not enough to confer community immunity. In the bottom box, a critical portion of the population is immunized, protecting most community members.
The principle of community immunity applies to control of a variety of contagious diseases, including influenza, measles, mumps, rotavirus, and pneumococcal disease.