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herd immunity

noun
  1. the immunity or resistance to a particular infection that occurs in a group of people or animals when a very high percentage of individuals have been vaccinated or previously exposed to the infection.
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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

herd immunity in Medicine

herd immunity

(hûrd)
n.
  1. Resistance to the spread of infectious disease in a group because susceptible members are few, making transmission from an infected member unlikely.
  2. The immunologic status of a population, determined by the ratio of resistant to susceptible members and their distribution.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.