Herd immunity

protection from infectious disease that occurs when a sufficient fraction of a population has become immune (through vaccination or previous infections)

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.[1][2]

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.

When many individuals have immunity, they do not spread the disease any more. This either stops or slows the spread of disease.[3]


  1. Fine, P.; Eames, K.; Heymann, D. L. (1 April 2011). "'Herd immunity': A rough guide". Clinical Infectious Diseases. 52 (7): 911–16. doi:10.1093/cid/cir007. PMID 21427399.
  2. Gordis, L. (2013). Epidemiology. Elsevier Health Sciences. pp. 26–27. ISBN 978-1455742516. Retrieved 29 March 2015.
  3. Merrill, R. M. (2013). Introduction to Epidemiology. Jones & Bartlett Publishers. pp. 68–71. ISBN 978-1449645175. Retrieved 29 March 2015.