Herd immunity
Herd immunity
Herd immunity (also known as population immunity) is a form of indirect protection from infectious disease that can occur with some diseases when a sufficient percentage of a population has become immune to an infection, whether through previous infections or vaccination, thereby reducing the likelihood of infection for individuals who lack immunity. Immune individuals are unlikely to contribute to disease transmission, disrupting chains of infection, which stops or slows the spread of disease. The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual.
Concept
In a population in which a large number of individuals are immune, chains of infection are likely to be disrupted, which either stops or slows the spread of disease. The larger the proportion of immune individuals, the more likely this is to happen. This effect is called herd immunity.
Mechanism
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, for example, is infectious but not contagious, so herd immunity does not apply.
Mathematical modeling
The percentage of immune individuals in a population above which a disease may no longer persist is the herd immunity threshold (HIT). Its value varies with the virulence of the disease, the efficacy of the vaccine, and the contact parameter for the population.
History
The term "herd immunity" was first used in 1923 by Topley and Wilson. It was recognized as a naturally occurring phenomenon in the 1930s when A. W. Hedrich published research on the epidemiology of measles in Baltimore, and took notice that after many children had become immune to measles, the number of new infections temporarily decreased, including among susceptible children.
See also
References
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