page_title: Herd immunity
section_title: History
hierarchical_section_title: Herd immunity / History
caption_attribution_description: English: measles cases in the USA between 1938 and 2019 1938-1943: https://www.census.gov/library/publications/1945/compendia/statab/66ed.html Part 2 page 89; 1944-1993: https://www.cdc.gov/mmwr/preview/index93.html MMVR October 21, 1994 / Vol. 42 / No. 53 page 67-72; https://www.cdc.gov/mmwr/preview/mmwrhtml/00035381.htm 1950-2016: CDC (file); 2017: https://wonder.cdc.gov/nndss/nndss_annual_tables_menu.asp?mmwr_year=2017; 2018: https://stacks.cdc.gov/view/cdc/82383 2010-2019: https://www.cdc.gov/measles/cases-outbreaks.html coverage rate: https://www.cdc.gov/vaccines/pubs/pinkbook/appendix/appdx-e.html
context_page_description: Herd immunity is a form of indirect protection from infectious disease that occurs when a sufficient percentage of a population has become immune to an infection, whether through vaccination or previous infections, thereby reducing the likelihood of infection for individuals who lack immunity. Immune individuals are unlikely to contribute to disease transmission, disrupting chains of infection, which stops or slows the spread of disease. The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual.
  Individuals can become immune by recovering from an earlier infection or through vaccination. Some individuals cannot become immune because of medical conditions, such as an immunodeficiency or immunosuppression, and for this group herd immunity is a crucial method of protection. Once the herd immunity threshold has been reached, disease gradually disappears from a population. This elimination, if achieved worldwide, may result in the permanent reduction in the number of infections to zero, called eradication.
context_section_description: Herd immunity was first recognized as a naturally occurring phenomenon in the 1930s when A. W. Hedrich published research on the epidemiology of measles in Baltimore, and took notice that after many children had become immune to measles, the number of new infections temporarily decreased, including among susceptible children. In spite of this knowledge, efforts to control and eliminate measles were unsuccessful until mass vaccination using the measles vaccine began in the 1960s. Mass vaccination, discussions of disease eradication, and cost–benefit analyses of vaccination subsequently prompted more widespread use of the term herd immunity. In the 1970s, the theorem used to calculate a disease's herd immunity threshold was developed. During the smallpox eradication campaign in the 1960s and 1970s, the practice of ring vaccination, of which herd immunity is integral to, began as a way to immunize every person in a "ring" around an infected individual to prevent outbreaks from spreading.
  Since the adoption of mass and ring vaccination, complexities and challenges to herd immunity have arisen. Modeling of the spread of infectious disease originally made a number of assumptions, namely that entire populations are susceptible and well-mixed, which is not the case in reality, so more precise equations have been developed. In recent decades, it has been recognized that the dominant strain of a microorganism in circulation may change due to herd immunity, either because of herd immunity acting as an evolutionary pressure or because herd immunity against one strain allowed another already-existing strain to spread. Emerging or ongoing fears and controversies about vaccination have reduced or eliminated herd immunity in certain communities, allowing preventable diseases to persist in or return to these communities.
caption: Measles cases in the United States before and after mass vaccination against measles began.
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