Herd immunity

For the evolutionary antiparasite defence, see Social 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 mass immunity) is a form of indirect protection that applies only to contagious diseases. It occurs when a sufficient percentage of a population has become immune to an infection, whether through previous infections or vaccination,[1] that the communicable pathogen cannot maintain itself in the population, its low incidence thereby reducing the likelihood of infection for individuals who lack immunity.[2][3][4]

Once the herd immunity has been reached, disease gradually disappears from a population and may result in eradication or permanent reduction of infections to zero if achieved worldwide.[5][6] Herd immunity created via vaccination has contributed to the reduction of many diseases.[7]

  1. ^ "Herd immunity | immunology". Encyclopedia Britannica. Archived from the original on 1 April 2023. Retrieved 13 June 2021.
  2. ^ Fine P, Eames K, Heymann DL (April 2011). ""Herd immunity": a rough guide". Clinical Infectious Diseases. 52 (7): 911–6. doi:10.1093/cid/cir007. PMID 21427399.
  3. ^ Gordis L (2013). Epidemiology. Elsevier Health Sciences. pp. 26–27. ISBN 978-1455742516. Archived from the original on 1 July 2023. Retrieved 29 March 2015.
  4. ^ "Cold-Causing Coronaviruses Don't Seem to Confer Lasting Immunity". The Scientist Magazine®. Archived from the original on 7 January 2021. Retrieved 26 January 2021.
  5. ^ Somerville M, Kumaran K, Anderson R (2012). Public Health and Epidemiology at a Glance. John Wiley & Sons. pp. 58–59. ISBN 978-1118308646. Archived from the original on 1 May 2021. Retrieved 29 March 2015.
  6. ^ Cliff, Smallman-Raynor M (2013). Oxford Textbook of Infectious Disease Control: A Geographical Analysis from Medieval Quarantine to Global Eradication. Oxford University Press. pp. 125–36. ISBN 978-0199596614. Archived from the original on 1 May 2021. Retrieved 29 March 2015.
  7. ^ Kim TH, Johnstone J, Loeb M (September 2011). "Vaccine herd effect". Scandinavian Journal of Infectious Diseases. 43 (9): 683–9. doi:10.3109/00365548.2011.582247. PMC 3171704. PMID 21604922.