herd immunity
noun
: a reduction in the risk of infection with a specific communicable disease (such as measles or influenza) that occurs when a significant proportion of the population has become immune to infection (as because of previous exposure or vaccination) so that susceptible individuals are much less likely to come in contact with infected individuals
… when the percentage of people vaccinated against a particular disease drops, herd immunity begins to fail.—
Katherine Hobson
Katherine HobsonSerious streptococcal disease does seem to be on the rise, however, perhaps because invasive strains are circulating against which we have not had time to develop much "herd immunity," as epidemiologists say.—Wayne Biddle
Wayne BiddleExamples of herd immunity in a Sentence
Recent Examples on the Web
But vaccinations have declined, resulting in a growing number of states no longer reporting rates consistent with herd immunity, and infections have returned.
—Mary Walrath-Holdridge, USA Today, 19 Aug. 2025
In 2000, the Centers for Disease Control and Prevention declared that measles had been effectively eliminated in the U.S., thanks to vaccine rates that hovered around 95%, the level needed for herd immunity.
—Michael R. Bloomberg, Mercury News, 25 July 2025
In fact, the United States’ vaccination rate has dropped below the 95% threshold necessary for herd immunity, when enough people are protected from infection that the virus can’t circulate in the community.
—Pittsburgh Post-Gazette, Twin Cities, 16 July 2025
Once vaccination rates fall below 95%, infectious disease specialists worry about the loss of herd immunity to protect people who are unvaccinated or cannot be vaccinated for health reasons such as a cancer diagnosis.
—Nicole Villalpando, Austin American Statesman, 2 July 2025
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Mary Walrath-Holdridge, USA Today, 19 Aug. 2025
In 2000, the Centers for Disease Control and Prevention declared that measles had been effectively eliminated in the U.S., thanks to vaccine rates that hovered around 95%, the level needed for herd immunity.
—Michael R. Bloomberg, Mercury News, 25 July 2025
In fact, the United States’ vaccination rate has dropped below the 95% threshold necessary for herd immunity, when enough people are protected from infection that the virus can’t circulate in the community.
—Pittsburgh Post-Gazette, Twin Cities, 16 July 2025
Once vaccination rates fall below 95%, infectious disease specialists worry about the loss of herd immunity to protect people who are unvaccinated or cannot be vaccinated for health reasons such as a cancer diagnosis.
—Nicole Villalpando, Austin American Statesman, 2 July 2025
See All Example Sentences for herd immunity
Cite this Entry
“Herd immunity.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/herd%20immunity. Accessed 23 Aug. 2025.
Copy CitationMedical Definition
herd immunity
noun
: a reduction in the probability of infection that is held to apply to susceptible members of a population in which a significant proportion of the individuals are immune because the chance of coming in contact with an infected individual is less
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