Herd immunity
historyHerd immunity (or community immunity) describes a type of immunity that occurs when the vaccination of a portion of the population (or herd) provides protection to unprotected individuals. Herd immunity theory proposes that, in diseases passed from person to person, it is more difficult to maintain a chain of infection when large numbers of a population are immune. The higher the proportion of individuals who are immune, the lower the likelihood that a susceptible person will come into contact with an infected individual.» History and Epidemiology of Global Smallpox Eradication From the training course titled "Smallpox: Disease, Prevention, and Intervention". The CDC and the World Health Organization. Slide 16-17.
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. Unvaccinated individuals are indirectly protected by vaccinated individuals, as the latter will not contract and transmit the disease between infected and susceptible individuals. 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.
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|+ Estimated Herd Immunity thresholds for vaccine preventable diseases
!align="center" width="150" |Disease||align="center" width="150" |Transmission||align="center" width="100" | R0||align="center" width="180" |Herd immunity threshold
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| align="center" |Diphtheria || align="center"|Saliva || align="center"|6-7 || align="center"| 85%
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| align="center" |Measles || align="center"| Airborne|| align="center"| 12-18|| align="center"|83 - 94%
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| align="center"| Mumps || align="center"| Airborne droplet||align="center"| 4-7|| align="center"| 75 - 86%
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| align="center"| Pertussis || align="center"|Airborne droplet || align="center"|12-17 || align="center"|92 - 94%
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| align="center"| Polio || align="center"|Fecal-oral route || align="center"| 5-7|| align="center"|80 - 86%
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| align="center"| Rubella || align="center"|Airborne droplet || align="center"| 5-7|| align="center"|80 - 85%
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| align="center"| Smallpox || align="center"| Social contact|| align="center"|6-7 || align="center"|83 - 85%
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| colspan="4" | - R0 is the basic reproduction number, or the average number of secondary infectious cases that are produced by a single index case in completely susceptible population.
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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. No vaccine offers complete protection, but the spread of disease from person to person is much higher in those who remain unvaccinated.
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. , 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.
Herd immunity only applies to diseases that are contagious. 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.