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Herd immunity

Herd 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 unvaccinated individuals.[1] 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.[2]

The effectiveness of a vaccine depends on the immune system reaction which the patient develops.[3] This involves the generation of long term memory B cells and T cells via adaptive immunity following innate immune responses. 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.[4] 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.[2] 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 immune disorder or for organ transplant recipients.

Estimated Herd Immunity thresholds for vaccine preventable diseases[2]
DiseaseTransmission R0[N]Herd immunity threshold
Diphtheria Saliva 6-7 85%
Measles Airborne 12-1883 - 94%
Mumps Airborne droplet 4-7 75 - 86%
Pertussis Airborne droplet 12-17 92 - 94%
Polio Fecal-oral route 5-780 - 86%
Rubella Airborne droplet 5-780 - 85%
Smallpox Social contact6-7 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.

Although no vaccine offers 100% protection, the spread of disease from person to person is much higher in those who remain un-vaccinated.[5] 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.[4] 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 controversy in the UK.) Additionally, some vaccines, such as the tetanus vaccine, protect only vaccinated persons from pathogens, and do not generate herd immunity.[6]

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


  1. ^ John TJ, Samuel R (2000). "Herd immunity and herd effect: new insights and definitions". Eur. J. Epidemiol. 16 (7): 601-6. PMID 11078115.
  2. ^ 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.
  3. ^ Fedson D (1998). "Measuring protection: efficacy versus effectiveness". Dev Biol Stand 95: 195–201. PMID 9855432.
  4. ^ a b Fine P (1993). "Herd immunity: history, theory, practice". Epidemiol Rev 15 (2): 265-302. PMID 8174658.
  5. ^ Jamison DT, Breman JG, Measham AR (editors). "Chapter 4: Vaccine-preventable Diseases", Priorities in Health: Disease Control Priorities Companion Volume. World Bank Publications. ISBN 0-8213-6260-7. 
  6. ^ 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. PMID 11773570.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Herd_immunity". A list of authors is available in Wikipedia.
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