Herd immunity

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The top box shows an outbreak in a community in which a few people are ill (shown in red) and the rest are healthy but unimmunized (shown in blue); the illness spreads freely through the population. The middle box shows the same population where a small number have been immunized (shown in yellow); those immunized are unaffected by the illness, but others are not. In the bottom box, a critical portion of the population have been immunized; this prevents the illness from spreading significantly, even to unimmunized people.

Herd immunity or herd effect, also called community immunity, describes a form of immunity[1] that occurs when the vaccination of a significant portion of a population provides a measure of protection for individuals who have not developed immunity.[2] 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 or less susceptible to the disease. The greater the proportion of individuals who are resistant, the smaller the probability that a susceptible individual will come into contact with an infectious individual.[3]


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.[4] Unvaccinated individuals are indirectly protected by vaccinated individuals, as the latter are less likely to contract and transmit the disease between infected and susceptible individuals.[3] 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, organ transplant recipients, or people with egg allergies.

Herd immunity generally applies only 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.[5] Nor does it apply to the IPV poliomyelitis vaccine that protects the individual from viremia and paralytic polio but does not prevent the fecal-oral spread of infection. 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.

Estimated Herd Immunity thresholds for vaccine preventable diseases[3]
Disease Transmission R0 Herd immunity threshold
Diphtheria Saliva 6–7 85%
Measles Airborne 12–18 83–94%
Mumps Airborne droplet 4–7 75–86%
Pertussis Airborne droplet 12–17 92–94%
Polio Fecal-oral route 5–7 80–86%
Rubella Airborne droplet 5–7 83–85%
Smallpox Social contact 6–7 83–85%
^ - 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.

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.[4] No vaccine offers complete protection, but the spread of disease from person to person is much higher in those who remain unvaccinated.[6] 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.[7][8][9]

A report by the Centers for Disease Control and Prevention analyzed the gastroenteritis hospitalization rate in the USA before (2000 to 2006) and after (2008 to 2010, the transition year 2007 is excluded) the infant rotavirus vaccination program was introduced in 2006. The analysis shows that the hospitalization rate was reduced not only among infants but also among older children while adults, who are not usually vaccinated against rotavirus, also saw significant declines in hospitalization rates. Since the rotavirus vaccine was introduced, hospitalizations due to rotavirus among children under 5 years of age have decreased by 80%, among those between 5 to 14 years old by 70%, and among people over 65 years of age by 14%. The decline in hospitalizations for people between 14 and 65 years of age fell somewhere in between. Rotavirus cases not requiring hospitalization were not measured, but a decline in cases can be reasonably deduced[citation needed]. This report demonstrates that herd immunity incurred by the infant rotavirus vaccination program benefits the rest of the population.[10][11]

Transmission in mixed populations v. social networks[edit]

The standard mathematical definition of herd immunity applies only to "well-mixed populations," in which each infected individual is capable of transmitting the disease to any susceptible individual, regardless of social ties or location. More specifically, the relationship between the basic reproduction number R0 and the herd immunity threshold relies on a calculation that is valid only in well-mixed populations. Actual large populations, however, are better described as social networks, in which transmission can occur only between peers and neighbors. The shape of a social network can alter the level of vaccination required for herd immunity, as well as the likelihood that a population will achieve herd immunity.[12][13] While social networks have a lower herd immunity threshold, in humans, the perception of this lower threshold can adversely affect voluntary inoculation rates, potentially making herd immunity more fragile.[14]

See also[edit]


  1. ^ "Community Immunity ("Herd" Immunity)". National Institute of Allergy and Infectious Diseases. Retrieved 7 April 2014. 
  2. ^ John TJ, Samuel R (2000). "Herd immunity and herd effect: new insights and definitions". Eur. J. Epidemiol. 16 (7): 601–6. doi:10.1023/A:1007626510002. PMID 11078115. 
  3. ^ a b c 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.
  4. ^ a b Fine P (1993). "Herd immunity: history, theory, practice". Epidemiol Rev 15 (2): 265–302. PMID 8174658. 
  5. ^ Fair E, Murphy T, Golaz A, Wharton M (2002). "Philosophic objection to vaccination as a risk for tetanus among children younger than 15 years". Pediatrics 109 (1): E2. doi:10.1542/peds.109.1.e2. PMID 11773570. 
  6. ^ Jamison DT, Breman JG, Measham AR, ed. (2006). "Chapter 4: Cost-Effective Strategies for the Excess Burden of Disease in Developing Countries
    Section: Vaccine-preventable Diseases"
    . Priorities in Health: Disease Control Priorities Companion Volume. World Bank Publications. ISBN 0-8213-6260-7.
  7. ^ Glanz JM, McClure DL, Magid DJ, et al. (June 2009). "Parental refusal of pertussis vaccination is associated with an increased risk of pertussis infection in children". Pediatrics 123 (6): 1446–51. doi:10.1542/peds.2008-2150. PMID 19482753. 
  8. ^ Gupta RK, Best J, MacMahon E (May 2005). "Mumps and the UK epidemic 2005". BMJ (Clinical Research Ed.) 330 (7500): 1132–5. doi:10.1136/bmj.330.7500.1132. PMC 557899. PMID 15891229. 
  9. ^ Salathé M, Bonhoeffer S (December 2008). "The effect of opinion clustering on disease outbreaks". J R Soc Interface. 5 (29): 1505–8. doi:10.1098/rsif.2008.0271. PMC 2607358. PMID 18713723. 
  10. ^ Knox, Richard (27 August 2013). "Vaccinating Babies For Rotavirus Protects The Whole Family". NPR. Retrieved 3 September 2013. 
  11. ^ Gastanaduy, Paul; Cums, Aaron; Parashar, Umesh; Lopman, Ben (2013). "Gastroenteritis Hospitalizations in Older Children and Adults in the United Sates Before and After Implementation of Infant Rotavirus Vaccination". The Journal of the American Medical Association 310 (8): 851–853. doi:10.1001/jama.2013.170800. 
  12. ^ Fu F., Rosenbloom D. I., Wang L., Nowak M. A. (2010). "Imitation dynamics of vaccination behaviour on social networks". Proceedings of the Royal Society B 278 (1702): 42–49. doi:10.1098/rspb.2010.1107. PMC 2992723. PMID 20667876. 
  13. ^ Perisic A., Bauch C. T. (2009). Meyers, Lauren Ancel, ed. "Social contact networks and disease eradicability under voluntary vaccination". PLoS Computational Biology 5 (2): e1000280. doi:10.1371/journal.pcbi.1000280. PMC 2625434. PMID 19197342. 
  14. ^ "Vaccine vacuum". Harvard Gazette. 2010-07-29. 

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