Editing Herd immunity (section)

==History==
[[File:Measles US 1938-2019.png|thumb|Measles cases in the [[United States]] before and after mass vaccination against measles began.]]

Herd immunity was recognized as a naturally occurring phenomenon in the 1930s when it was observed that after a significant number of children had become immune to [[measles]], the number of new infections temporarily decreased.<ref name="pmid15106084">* {{cite journal |vauthors=Hinman AR, Orenstein WA, Papania MJ |date=May 2004 |title=Evolution of measles elimination strategies in the United States |journal=The Journal of Infectious Diseases |volume=189 |issue=Suppl 1 |pages=S17-22 |doi=10.1086/377694 |pmid=15106084 |doi-access=free}}<br />*{{cite journal |vauthors=Sencer DJ, Dull HB, Langmuir AD |date=March 1967 |title=Epidemiologic basis for eradication of measles in 1967 |journal=Public Health Reports |volume=82 |issue=3 |pages=253–6 |doi=10.2307/4592985 |jstor=4592985 |pmc=1919891 |pmid=4960501}}</ref> Mass vaccination to induce herd immunity has since become common and proved successful in preventing the spread of many contagious diseases.<ref name="pmid15627236" /> [[Vaccine hesitancy|Opposition to vaccination]] has posed a challenge to herd immunity, allowing preventable diseases to persist in or return to populations with inadequate vaccination rates.<ref name="pmid22926181" /><ref name="pmid23584253" /><ref name="pmid23807359" />

The exact herd immunity threshold (HIT) varies depending on the [[basic reproduction number]] of the disease. An example of a disease with a high threshold was the measles, with a HIT exceeding 95%.<ref>{{cite journal |vauthors=van Boven M, Kretzschmar M, Wallinga J, O'Neill PD, Wichmann O, Hahné S |date=November 2010 |title=Estimation of measles vaccine efficacy and critical vaccination coverage in a highly vaccinated population |journal=Journal of the Royal Society, Interface |volume=7 |issue=52 |pages=1537–44 |doi=10.1098/rsif.2010.0086 |pmc=2988255 |pmid=20392713}}</ref>

The term "herd immunity" was first used in 1894 by American veterinary scientist and then Chief of the Bureau of Animal Industry of the US Department of Agriculture [[Daniel Elmer Salmon]] to describe the healthy vitality and resistance to disease of well-fed herds of hogs. In 1916 veterinary scientists inside the same Bureau of Animal Industry used the term to refer to the immunity arising following recovery in cattle infected with brucellosis, also known as "contagious abortion." By 1923 it was being used by British bacteriologists to describe experimental epidemics with mice, experiments undertaken as part of efforts to model human epidemic disease. By the end of the 1920s the concept was used extensively - particularly among British scientists - to describe the build up of immunity in populations to diseases such as diphtheria, scarlet fever, and influenza.<ref>{{cite journal |last1=Robertson |first1=David |title=Of Mice and Schoolchildren: A Conceptual History of Herd Immunity |journal=American Journal of Public Health |date=10 June 2021 |volume=111 |issue=8 |pages=e1–e8 |doi=10.2105/AJPH.2021.306264 |pmid=34111938 |pmc=8489650 |s2cid=235404539 }}</ref> Herd immunity was 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.<ref>{{cite journal | author = Hedrich AW | year = 1933 | title = Monthly Estimates of the Child Population Susceptible to Measles, 1900–1931, Baltimore, Md. | url = | journal = American Journal of Epidemiology | volume = 17 | issue = 3| pages = 613–636 | doi = 10.1093/oxfordjournals.aje.a117929 }}</ref><ref name="pmid15106084" /> In spite of this knowledge, efforts to control and eliminate measles were unsuccessful until mass vaccination using the [[measles vaccine]] began in the 1960s.<ref name="pmid15106084" /> Mass vaccination, discussions of disease eradication, and cost–benefit analyses of vaccination subsequently prompted more widespread use of the term ''herd immunity''.<ref name="pmid21427399" /> In the 1970s, the theorem used to calculate a disease's herd immunity threshold was developed.<ref name="pmid21427399" /> During the [[Smallpox#Eradication|smallpox eradication campaign]] in the 1960s and 1970s, the practice of ''[[ring vaccination]]'', to which herd immunity is integral, began as a way to immunize every person in a "ring" around an infected individual to prevent outbreaks from spreading.<ref>{{cite journal|vauthors=Strassburg MA|title=The global eradication of smallpox|journal=American Journal of Infection Control|volume=10|issue=2|pages=53–9|date=May 1982|pmid=7044193|doi=10.1016/0196-6553(82)90003-7}}</ref>

Since the adoption of mass and ring vaccination, complexities and challenges to herd immunity have arisen.<ref name=pmid21427399/><ref name="pmid22561998"/> Modeling of the spread of contagious 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.<ref name=pmid21427399/> 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.<ref name=pmid21310617/><ref name=pmid22903767/> 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.<ref name=pmid22926181/><ref name=pmid23584253/><ref name=pmid23807359/>