Herd immunity 87175 220066293 2008-06-18T03:19:32Z DOI bot 6652755 Citation maintenance. Initiated by [[User:Fconaway|Fconaway]]. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. '''Herd immunity''' (or '''community immunity''') describes a type of [[immunity (medical)|immunity]] that occurs when the [[vaccination]] of a portion of the [[population]] (or herd) provides protection to unvaccinated individuals.<ref>{{cite journal |author=John TJ, Samuel R |title=Herd immunity and herd effect: new insights and definitions |journal=Eur. J. Epidemiol. |volume=16 |issue=7 |pages=601–6 |year=2000 |pmid=11078115| doi = 10.1023/A:1007626510002}}</ref> 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 more immune individuals present in a population, the lower the likelihood that a susceptible person will come into contact with an infected individual.<ref name= Smallpox>[http://www.bt.cdc.gov/agent/smallpox/training/overview/pdf/eradicationhistory.pdf 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.</ref> The effectiveness of a [[vaccine]] depends on the [[immune system]] reaction which the patient develops.<ref name=Fedson_1998>{{cite journal |author=Fedson D |title=Measuring protection: efficacy versus effectiveness |journal=Dev Biol Stand |volume=95 |pages=195&ndash;201 |year=1998 |pmid=9855432 }}</ref> This involves the generation of long term memory [[B cell]]s and [[T cell]]s via [[adaptive immunity]] following [[innate immune response]]s. Sometimes the antigen contained in the vaccine doesn't trigger an immune response. In the latter case there is need for new and stronger vaccines. Vaccination acts as a sort of "[[firebreak]]" in the spread of the [[disease]], slowing or preventing further transmission of the disease to others.<ref name= Fine>{{cite journal |author=Fine P |title=Herd immunity: history, theory, practice |journal=Epidemiol Rev |volume=15 |issue=2 |pages=265–302 |year=1993 |pmid=8174658}}</ref> For example, if Person A had a disease and exposed Person B who was immune because of vaccination, Person B would not get ill and could not pass on the disease to Person C when he comes into contact with him. So even if Person C is not vaccinated, he indirectly gets protection from the disease.<ref name= Smallpox/> Hence herd immunity may be used to reduce spread of an illness and to protect a vulnerable, un-vaccinated subgroup. However because only a small fraction of the population (or herd) can be left un-vaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines due a medical condition such as an [[immunodeficiency|immune disorder]] or for [[organ transplant]] recipients. {| class = "prettytable" style = "float:right; font-size:85%; margin-left:15px" |+ '''Estimated Herd Immunity thresholds for vaccine preventable diseases'''<ref name=Smallpox /> !align="center" width="150" |Disease||align="center" width="150" |Transmission||align="center" width="100" | R<sub>0</sub>||align="center" width="180" |Herd immunity threshold |- | align="center" |[[Diphtheria]] || align="center"|Saliva || align="center"|6-7 || align="center"| 85% |- | align="center" |[[Measles]] || align="center"| Airborne|| align="center"| 12-18|| align="center"|83 - 94% |- | align="center"| [[Mumps]] || align="center"| Airborne droplet||align="center"| 4-7|| align="center"| 75 - 86% |- | align="center"| [[Pertussis]] || align="center"|Airborne droplet || align="center"|12-17 || align="center"|92 - 94% |- | align="center"| [[Polio]] || align="center"|Fecal-oral route || align="center"| 5-7|| align="center"|80 - 86% |- | align="center"| [[Rubella]] || align="center"|Airborne droplet || align="center"| 5-7|| align="center"|80 - 85% |- | align="center"| [[Smallpox]] || align="center"| Social contact|| align="center"|6-7 || align="center"|83 - 85% |- | colspan="4" | <small>{{note|R}} - R<sub>0</sub> 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.</small> |} Although no vaccine offers 100% protection, the spread of disease from person to person is much higher in those who remain un-vaccinated.<ref>{{cite book |author= Jamison DT, Breman JG, Measham AR (editors) |title=Priorities in Health: Disease Control Priorities Companion Volume |publisher=World Bank Publications |location= |year= |pages= |chapter= Chapter 4: Vaccine-preventable Diseases |chapterurl= http://0-www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=vaccine+AND+pih%5Bbook%5D+AND+600051%5Buid%5D&rid=pih.section.136#151 |isbn=0-8213-6260-7}}</ref> [[Virology|Virologists]] have found that when a certain percentage of a population is vaccinated, the spread of the disease is effectively stopped. This critical percentage, called the ''herd immunity threshold'', depends on the disease and the vaccine.<ref name= Fine/> It is the general aim of those involved in [[public health]] to establish herd immunity in most populations. However complications arise when wide spread vaccination is not possible, and when vaccines fail (See the [[MMR vaccine]] [[MMR vaccine controversy|controversy]] in the [[United Kingdom|UK]].) Additionally, some vaccines, such as the [[tetanus]] vaccine, protect only vaccinated persons from [[pathogen]]s, and do not generate herd immunity.<ref>{{cite journal |author=Fair E, Murphy T, Golaz A, Wharton M |title=Philosophic objection to vaccination as a risk for tetanus among children younger than 15 years |url= http://pediatrics.aappublications.org/cgi/content/full/109/1/e2 |journal=Pediatrics |volume=109 |issue=1 |pages=E2 |year=2002 |pmid=11773570 | doi = 10.1542/peds.109.1.e2}}</ref> 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. ==See also== *[[Mathematical modelling in epidemiology#The mathematics of mass vaccination|The mathematics of mass vaccination]] ==References== <!-- --------------------------------------------------------------- See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for a discussion of different citation methods and how to generate footnotes using the <ref> & </ref> tags and the {{Reflist}} template -------------------------------------------------------------------- --> {{Reflist|2}} ==External links== *[http://www.pitt.edu/~super1/lecture/lec1181/001.htm Herd immunity lecture] slide show *[http://www.mathepi.com/maindir/herd.html Mathematical epidemiology information] [[Category:Epidemiology]] [[Category:Prevention]] [[de:Herdenimmunität]] [[nl:kudde-immuniteit]]