- 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 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.[2]
Estimated Herd Immunity thresholds for vaccine preventable diseases[2] 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 80 - 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. 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[update], 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.
Contents
In social networks
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 illustrated in the table above 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/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.[9][10] Compared to well-mixed populations, herd immunity in social networks is particularly fragile.[11]
See also
References
- ^ 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.
- ^ 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.
- ^ a b Fine P (1993). "Herd immunity: history, theory, practice". Epidemiol Rev 15 (2): 265–302. PMID 8174658.
- ^ 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. http://www.ncbi.nlm.nih.gov/books/NBK10258/#A151. - ^ 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. http://pediatrics.aappublications.org/cgi/pmidlookup?view=long&pmid=19482753.
- ^ 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. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=557899.
- ^ 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. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2607358.
- ^ 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. http://pediatrics.aappublications.org/cgi/content/full/109/1/e2.
- ^ 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. http://rspb.royalsocietypublishing.org/content/early/2010/07/26/rspb.2010.1107.
- ^ 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. http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.1000280.
- ^ "Vaccine vacuum". Harvard Gazette. 2010-07-29. http://news.harvard.edu/gazette/story/2010/07/vaccine-vacuum/.
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