For the evolutionary antiparasite defence, see Social immunity.
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] 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]
^Gordis L (2013). Epidemiology. Elsevier Health Sciences. pp. 26–27. ISBN978-1455742516. Archived from the original on 1 July 2023. Retrieved 29 March 2015.