Community Immunity (also known as “Herd Immunity”)
When a critical portion of a community is immunized against a particular disease, most members of the community are protected because there is little opportunity for a disease outbreak. In the illustration below, the top box depicts a community in which no one is immunized against influenza and an outbreak occurs. In the middle box, a portion of the population is immunized, but it is not enough to confer community immunity. The bottom box shows that immunizing a critical portion of the population confers protection for most community members. Often, even those who are not immunized are protected because community immunity has been achieved and the spread of contagious diseases such as influenza is contained.
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