Herd immunity

herd immunity § Mechanismthe wider community
Herd immunity (also called herd effect, community immunity, population immunity, or social immunity) is a form of indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, thereby providing a measure of protection for individuals who are not immune.wikipedia
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Vaccination

vaccinationsvaccinatedvaccinating
Individual immunity can be gained through recovering from a natural infection or through artificial means such as vaccination.
When a sufficiently large percentage of a population has been vaccinated, herd immunity results.

Measles

rubeolameasles virusAcute Measles encephalitis
It was recognized as a naturally occurring phenomenon in the 1930s when it was observed that after a significant number of children had become immune to measles, the number of new infections temporarily decreased, including among susceptible children.
To achieve herd immunity, more than 95% of the community must be vaccinated due to the ease with which measles is transmitted from person to person.

Infection

infectious diseaseinfectious diseasesinfectious
Mass vaccination to induce herd immunity has since become common and proved successful in preventing the spread of many infectious diseases.
There is also the phenomenon of herd immunity which offers a measure of protection to those otherwise vulnerable people when a large enough proportion of the population has acquired immunity from certain infections.

Influenza vaccine

flu vaccineinfluenzainfluenza vaccination
Influenza (flu) is more severe in the elderly than in younger age groups, but influenza vaccines lack effectiveness in this demographic due to a waning of the immune system with age.
This is in contrast with the high herd immunity strategies for other infectious diseases such as polio and measles.

West African Ebola virus epidemic

Ebola outbreak2014 Ebola outbreak2014 West Africa Ebola outbreak
While the vaccine had shown 100% efficacy in individuals, more conclusive evidence was needed regarding its capacity to protect populations through herd immunity.

Vaccination policy

compulsory vaccinationmandatory vaccinationimmunization
Mandatory vaccination may be beneficial to eradication efforts if not enough people choose to get vaccinated.
Besides individual protection from getting ill, some vaccination policies also aim to provide the community as a whole with herd immunity.

Externality

externalitiesnegative externalitiesnegative externality
It is regarded as a positive externality of high levels of immunity, producing an additional benefit of disease reduction that would not occur had no herd immunity been generated in the population.
For example, individuals who are vaccinated reduce the risk of contracting the relevant disease for all others around them, and at high levels of vaccination, society may receive large health and welfare benefits (herd immunity); but any one individual can refuse vaccination, still avoiding the disease by "free riding" on the costs borne by others.

Poliomyelitis

polioinfantile paralysisparalytic polio
Eradication efforts that rely on herd immunity are currently underway for poliomyelitis, though civil unrest and distrust of modern medicine have made this difficult.
Both types induce immunity to polio, efficiently blocking person-to-person transmission of wild poliovirus, thereby protecting both individual vaccine recipients and the wider community (so-called herd immunity).

Pneumococcal vaccine

pneumococcalpneumococcal vaccinationStreptococcus pneumoniae
Since then, pneumococcal vaccines that provide protection from the emerging serotypes have been introduced and have successfully countered their emergence.
Due to the indirect protection conferred by herd immunity, a significant decline in IPD in children and in unvaccinated adults has also been shown.

Rubella

German measlesdisease of the same namematernal rubella
Universal immunisation producing a high level of herd immunity is important in the control of epidemics of rubella.

Vaccine controversies

anti-vaccinationanti-vaccination movementanti-vaccine
Emerging or ongoing vaccine controversies and various reasons for opposing vaccination have reduced or eliminated herd immunity in certain communities, allowing preventable diseases to persist in or return to these communities.
Incomplete vaccine coverage increases the risk of disease for the entire population, including those who have been vaccinated, because it reduces herd immunity.

Ring vaccination

ring vaccinate
During the smallpox eradication campaign in the 1960s and 1970s, the practice of ring vaccination, of which herd immunity is integral to, began as a way to immunize every person in a "ring" around an infected individual to prevent outbreaks from spreading.
Herd immunity

Immunity (medical)

immunityimmuneimmune response
Herd immunity (also called herd effect, community immunity, population immunity, or social immunity) is a form of indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, thereby providing a measure of protection for individuals who are not immune.

Smallpox

small poxsmall-poxvariola
During the smallpox eradication campaign in the 1960s and 1970s, the practice of ring vaccination, of which herd immunity is integral to, began as a way to immunize every person in a "ring" around an infected individual to prevent outbreaks from spreading. This method was used for the eradication of smallpox in 1977 and for the regional elimination of other diseases.

Contagious disease

contagiouscontagionnon-contagious disease
Herd immunity does not apply to all diseases, just those that are contagious, meaning that they can be transmitted from one individual to another.

Tetanus

lockjawanti-tetanuslock jaw
Tetanus, for example, is infectious but not contagious, so herd immunity does not apply.

Susceptible individual

susceptiblesusceptibilitydisease susceptibility
It was recognized as a naturally occurring phenomenon in the 1930s when it was observed that after a significant number of children had become immune to measles, the number of new infections temporarily decreased, including among susceptible children.

HIV/AIDS

AIDSHIVacquired immune deficiency syndrome
Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.

Lymphoma

lymphomaslymphatic cancerlymphosarcoma
Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.

Leukemia

leukaemialeukemiasleukemic
Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.

Bone marrow

marrowred bone marrowbone marrow stroma
Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.

Spleen

splenicsplenetichilum
Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.

Chemotherapy

chemotherapeuticantineoplasticantineoplastic agent
Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.

Radiation therapy

radiotherapyradiationradiation oncology
Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.