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What is Herd Immunity?

Herd immunity is a form of indirect immunity against infectious diseases, such as measles or chickenpox.

Herd immunity occurs when a large percentage of a population has become immune to a disease, either by naturally recovering from the disease or receiving a vaccination, thereby providing a measure of protection for individuals who are not immune.

If a large number of individuals in a community are immune, the spread of infection either stops of slows down. The greater the proportion of immune individuals in a community, the smaller the probability that those who are not immune will come into contact with an infectious individual. However, if immunisation rates fall (i.e., individuals stop receiving vaccinations), herd immunity can break down and lead to an increase in the number of diseased cases.

Video on Herd Immunity

Learn more about how herd immunity works, with this easy-to-follow video:

Vaccines and Herd Immunity

Paul Andersen explains how immune individuals in a population give the entire group herd immunity. Concepts of immunity, vaccines, basic reproduction number, and herd immunity threshold are discussed.

Search the Bozeman Science Link to external website website for more information on vaccines and vaccination.


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