herd immunity herd 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, thereby reducing the likelihood of infection for individuals who lack immunity. for the evolutionary antiparasite defence, see social immunity. 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. 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 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. in addition to not being immune, individuals in one of these groups may be at a greater risk of developing complications from infection because of their medical status, but they may still be protected if a large enough percentage of the population is immune. high levels of immunity in one age group can create herd immunity for other age groups. vaccinating adults against pertussis reduces pertussis incidence in infants too young to be vaccinated, who are at the greatest risk of complications from the disease. this is especially important for close family members, who account for most of the transmissions to young infants. in the same manner, children receiving vaccines against pneumococcus reduces pneumococcal disease incidence among younger, unvaccinated siblings. vaccinating children against pneumococcus and rotavirus has had the effect of reducing 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-age 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. for sexually transmitted infections (stis), high levels of immunity in heterosexuals of one sex induces herd immunity for heterosexuals of both sexes. 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. high-risk behaviors make eliminating stis difficult because, even though most infections occur among individuals with moderate risk, the majority of transmissions occur because of individuals who engage in high-risk behaviors. for this reason, in certain populations it may be necessary to immunize high-risk individuals regardless of gender. evolutionary pressure and serotype replacement herd immunity itself acts as an evolutionary pressure on pathogens, influencing viral evolution by encouraging the production of novel strains, referred to as escape mutants, that are able to evade herd immunity and infect previously immune individuals. 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 there are more strains 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. for both influenza and norovirus, epidemics temporarily induce herd immunity until a new dominant strain emerges, causing successive waves of epidemics. as this evolution poses a challenge to herd immunity, broadly neutralizing antibodies and "universal" vaccines that can provide protection beyond a specific serotype are in development. 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). this did not result in a proportionate increase in disease incidence though, since nvts were less invasive than vts. since then, pneumococcal vaccines that provide protection from the emerging serotypes have been introduced and have successfully countered their emergence. the possibility of future shifting remains, so further strategies to deal with this include expansion of vt coverage and the development of vaccines that use either killed whole-cells, which have more surface antigens, or proteins present in multiple serotypes. eradication of diseases 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 eliminated – no more endemic transmissions occur. if elimination is achieved worldwide and the number of cases is permanently reduced to zero, then a disease can be declared eradicated. eradication can thus be considered the final effect or end-result of public health initiatives to control the spread of infectious disease. in cases in which herd immunity is compromised, on the contrary, disease outbreaks among the unvaccinated population are likely to 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. to date, two diseases have been eradicated using herd immunity and vaccination: rinderpest and smallpox. 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. as the number of free riders in a population increases, outbreaks of preventable diseases become more common and more severe due to loss of herd immunity. individuals may choose to free ride or be hesitant to vaccinate for a variety of reasons, including the belief that vaccines are ineffective, or that the risks associated with vaccines are greater than those associated with infection, mistrust of vaccines or public health officials, bandwagoning or groupthinking, social norms or peer pressure, and religious beliefs. certain individuals are more likely to choose not to receive vaccines if vaccination rates are high enough to convince a person that he or she may not need to be vaccinated, since a sufficient percentage of others are already immune. mechanism individuals who are immune to a disease act as a barrier in the spread of disease, slowing or preventing the transmission of disease to others. an individual's immunity can be acquired via a natural infection or through artificial means, such as vaccination. when a critical proportion of the population becomes immune, called the herd immunity threshold (hit) or herd immunity level (hil), the disease may no longer persist in the population, ceasing to be endemic. the theoretical basis for herd immunity generally assumes that vaccines induce solid immunity, that populations mix at random, that the pathogen does not evolve to evade the immune response, and that there is no non-human vector for the disease. theoretical basis