thumb|upright=1.5|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.

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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, that the communicable pathogen cannot maintain itself in the population, its low incidence thereby reducing the likelihood of infection for individuals who lack immunity.

<!-- Methods -->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. Herd immunity created via vaccination has contributed to the reduction of many diseases.

Effects

Protection of those without immunity

thumb|Herd immunity protects vulnerable communities.

Some individuals either cannot develop immunity after vaccination or for medical reasons cannot be vaccinated. Newborn infants are too young to receive many vaccines, either for safety reasons or because passive immunity renders the vaccine ineffective. Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had, and vaccines may not be of any use for them because of their immunodeficiency.

A portion of those vaccinated may not develop long-term immunity. Vaccine contraindications may prevent certain individuals from being vaccinated.

High levels of immunity in one age group can create herd immunity for other age groups. This is especially important for close family members, who account for most of the transmissions to young infants. Vaccinating children against pneumococcus and rotavirus has reduced pneumococcus- and rotavirus-attributable hospitalizations for older children and adults, who do not normally receive these vaccines. Influenza (flu) is more severe in the elderly than in younger age groups, but influenza vaccines lack effectiveness in this demographic due to a waning of the immune system with age. The prioritization of school-aged children for seasonal flu immunization, which is more effective than vaccinating the elderly, however, has been shown to create a certain degree of protection for the elderly. Vaccines against STIs that are targeted at heterosexuals of one sex result in significant declines in STIs in heterosexuals of both sexes if vaccine uptake in the target sex is high. Herd immunity from female vaccination does not, however, extend to males who have sex with males. The evolution of new strains is known as serotype replacement, or serotype shifting, as the prevalence of a specific serotype declines due to high levels of immunity, allowing other serotypes to replace it.

At the molecular level, viruses escape from herd immunity through antigenic drift, which is when mutations accumulate in the portion of the viral genome that encodes for the virus's surface antigen, typically a protein of the virus capsid, producing a change in the viral epitope. Alternatively, the reassortment of separate viral genome segments, or antigenic shift, which is more common when more strains are in circulation, can also produce new serotypes. When either of these occur, memory T cells no longer recognize the virus, so people are not immune to the dominant circulating strain.

Initial vaccines against Streptococcus pneumoniae significantly reduced nasopharyngeal carriage of vaccine serotypes (VTs), including antibiotic-resistant types, only to be entirely offset by increased carriage of non-vaccine serotypes (NVTs).

Eradication of diseases

right|thumb|A cow with [[rinderpest in the "milk fever" position, 1982. The last confirmed case of rinderpest occurred in Kenya in 2001, and the disease was officially declared eradicated in 2011.]]

If herd immunity has been established and maintained in a population for a sufficient time, the disease is inevitably eliminatedno more endemic transmissions occur.

The benefits of eradication include ending all morbidity and mortality caused by the disease, financial savings for individuals, health care providers, and governments, and enabling resources used to control the disease to be used elsewhere. Eradication efforts that rely on herd immunity are currently underway for poliomyelitis, though civil unrest and distrust of modern medicine have made this difficult. Mandatory vaccination may be beneficial to eradication efforts if not enough people choose to get vaccinated.

Free riding

Herd immunity is vulnerable to the free rider problem. Individuals who lack immunity, including those who choose not to vaccinate, free ride off the herd immunity created by those who are immune. bandwagoning or groupthinking, social norms or peer pressure, and religious beliefs. An individual's immunity can be acquired via a natural infection or through artificial means, such as vaccination. If a population is immune to a disease in excess of that disease's HIT, the number of cases reduces at a faster rate, outbreaks are even less likely to happen, and outbreaks that occur are smaller than they would be otherwise. and then the disease is neither in a steady state nor decreasing in incidence, but is actively spreading through the population and infecting a larger number of people than usual.

In heterogeneous populations, R<sub>0</sub> is considered to be a measure of the number of cases generated by a "typical" contagious person, which depends on how individuals within a network interact with each other.

Boosts

Vaccination

The primary way to boost levels of immunity in a population is through vaccination. Vaccination is originally based on the observation that milkmaids exposed to cowpox were immune to smallpox, so the practice of inoculating people with the cowpox virus began as a way to prevent smallpox.

The immune system does not distinguish between natural infections and vaccines, forming an active response to both, so immunity induced by vaccination is similar to what would have occurred from contracting and recovering from the disease. To achieve herd immunity through vaccination, vaccine manufacturers aim to produce vaccines with low failure rates, and policy makers aim to encourage their use.

Assuming a vaccine is 100% effective, then the equation used for calculating the herd immunity threshold can be used for calculating the vaccination level needed to eliminate a disease, written as V<sub>c</sub>. 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 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. 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. 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. Mass vaccinations to induce herd immunity have since become common and proved successful in preventing the spread of many contagious diseases.

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.

Since the adoption of mass and ring vaccination, complexities and challenges to herd immunity have arisen. tags and the template

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  • A visual simulation of herd immunity written by Shane Killian and modified by Robert Webb