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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Herd immunity | 4/4 | https://en.wikipedia.org/wiki/Herd_immunity | reference | science, encyclopedia | 2026-05-05T07:29:16.564067+00:00 | kb-cron |
As seen from this equation, all other things being equal ("ceteris paribus"), any increase in either vaccine coverage or vaccine effectiveness, including any increase in excess of a disease's HIT, further reduces the number of cases of a disease. The rate of decline in cases depends on a disease's R0, with diseases with lower R0 values experiencing sharper declines. Vaccines usually have at least one contraindication for a specific population for medical reasons, but if both effectiveness and coverage are high enough, then herd immunity can protect these individuals. Vaccine effectiveness is not always adversely affected by passive immunity, so additional doses are recommended for some vaccines, while others are not administered until after an individual has lost his or her passive immunity.
=== Passive immunity ===
Individual immunity can also be gained passively, when antibodies to a pathogen are transferred from one individual to another. This can occur naturally, whereby maternal antibodies, primarily immunoglobulin G antibodies, are transferred across the placenta and in colostrum to fetuses and newborns. Passive immunity can also be gained artificially, when a susceptible person is injected with antibodies from the serum or plasma of an immune person. Protection generated from passive immunity is immediate, but wanes over the course of weeks to months, so any contribution to herd immunity is temporary. For diseases that are especially severe among fetuses and newborns, such as influenza and tetanus, pregnant women may be immunized to transfer antibodies to the child. In the same way, high-risk groups that are either more likely to experience infection or are more likely to develop complications from infection may receive antibody preparations to prevent these infections or to reduce the severity of symptoms.
== Cost–benefit analysis == Herd immunity is often accounted for when conducting cost–benefit analyses of vaccination programs. It is regarded as a positive externality of high levels of immunity, producing an additional benefit of disease reduction that would not occur had no herd immunity been generated in the population. Therefore, herd immunity's inclusion in cost–benefit analyses results both in more favorable cost-effectiveness or cost–benefit ratios, and an increase in the number of disease cases averted by vaccination. Study designs done to estimate herd immunity's benefit include recording disease incidence in households with a vaccinated member, randomizing a population in a single geographic area to be vaccinated or not, and observing the incidence of disease before and after beginning a vaccination program. From these, disease incidence may be seen to decrease to a level beyond what can be predicted from direct protection alone, indicating that herd immunity contributed to the reduction. Serotype replacement, when accounted for, reduces the predicted benefits of vaccination.
== History ==
Herd immunity was recognized as a naturally occurring phenomenon in the 1930s, when after a significant number of children had become immune to measles, the number of new infections was seen to decrease temporarily. Mass vaccinations to induce herd immunity have since become common and proved successful in preventing the spread of many contagious diseases. Opposition to vaccination has posed a challenge to herd immunity, allowing preventable diseases to persist in or return to populations with inadequate vaccination rates. 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%. The term "herd immunity" was first used in 1894 by American veterinary scientist and then Chief of the Bureau of Animal Industry (BIA) 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 BIA 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, tests 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 buildup of immunity in populations to diseases such as diphtheria, scarlet fever, and influenza. 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. 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, 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. Since the adoption of mass and ring vaccination, complexities and challenges to herd immunity have arisen. 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. In recent decades, the dominant strain of a microorganism in circulation has been recognized to change, possibly 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.
== See also == Premunity Social distancing
== Notes ==
== References ==
== External links ==
Topley WW, Wilson GS (May 1923). "The Spread of Bacterial Infection. The Problem of Herd-Immunity". The Journal of Hygiene. 21 (3): 243–9. doi:10.1017/s0022172400031478. PMC 2167341. PMID 20474777. A visual simulation of herd immunity written by Shane Killian and modified by Robert Webb Herd immunity simulation