herd immunity threshold

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An important concept in infectious disease epidemiology, this is the proportion of immune persons in a population, above which the incidence of new cases of a specific infection declines and eventually ceases altogether. This threshold varies according to the characteristics of the infectious pathogen. It is expressed mathematically as


where H is the herd immunity threshold, R0 is the basic reproductive rate, g is the infection transmission parameter, and T is the total population.

Subjects: Public Health and Epidemiology.

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