Herd immunity

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Herd immunity (or community immunity) describes a form of immunity that occurs when the vaccination of a significant portion of a population (or herd) provides a measure of protection for individuals who have not developed immunity.[1] Herd immunity theory proposes that, in contagious diseases that are transmitted from individual to individual, chains of infection are likely to be disrupted when large numbers of a population are immune to the disease. The greater the proportion of individuals who are immune, the smaller the probability that a susceptible individual will come into contact with an infectious individual.[2]

Vaccination acts as a sort of firebreak or firewall in the spread of the disease, slowing or preventing further transmission of the disease to others.[3] Unvaccinated individuals are indirectly protected by vaccinated individuals, as the latter will not contract and transmit the disease between infected and susceptible individuals.[2] Hence, a public health policy of herd immunity may be used to reduce spread of an illness and provide a level of protection to a vulnerable, unvaccinated subgroup. Since only a small fraction of the population (or herd) can be left unvaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines because of a medical condition such as an immune disorder or for organ transplant recipients.

The proportion of immune individuals in a population above which a disease may no longer persist is the herd immunity threshold. Its value varies with the virulence of the disease, the efficacy of the vaccine, and the contact parameter for the population.[3] No vaccine offers complete protection, but the spread of disease from person to person is much higher in those who remain unvaccinated.[4] It is the general aim of those involved in public health to establish herd immunity in most populations. Complications arise when widespread vaccination is not possible or when vaccines are rejected by a part of the population. As of 2009, herd immunity is compromised in some areas for some vaccine-preventable diseases, including pertussis and measles and mumps, in part because of parental refusal of vaccination.[5][6][7]

Herd immunity only applies to diseases that are contagious. It does not apply to diseases such as tetanus (which is infectious, but is not contagious), where the vaccine protects only the vaccinated person from disease.[8] Herd immunity should not be confused with contact immunity, a related concept wherein a vaccinated individual can 'pass on' the vaccine to another individual through contact.

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