Indirect protection from an infectious disease that occurs when a sufficiently large proportion of a population is immune, making transmission less likely.
When many people are immune, an infectious agent has fewer susceptible people to infect and transmit through. This can reduce the likelihood that an infectious chain will reach someone who is not immune.
The level of immunity needed to substantially reduce transmission varies between infections and depends partly on how easily the infectious agent spreads and how effective immunity is at preventing infection and transmission.
QUICK FACT
Herd immunity does not mean that everyone in a population is immune.
COMMONLY CONFUSED WITH
Immunity: an individual’s ability to mount a protective response; herd immunity describes an indirect population-level effect resulting from immunity being sufficiently common.
SEE ALSO