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

Serious 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

Examples of herd immunity in a Sentence

Recent Examples on the Web

It was discovered in the Netherlands in 2001 but is believed to have circulated long before then—so people all over the world already have some herd immunity. —Daniel R. Depetris, Newsweek, 10 Jan. 2025

For Measles, herd immunity occurs when 95% of the population is vaccinated. —Omer Awan, Forbes, 22 Dec. 2024

As for Makary, his most infamous take involved a February 2021 prediction that the United States would reach herd immunity within two months. —Benjamin Mazer, The Atlantic, 25 Nov. 2024

In communities that fail to meet the high threshold for herd immunity, the virus spreads easily and quickly. —Saima S. Iqbal, Scientific American, 26 Sep. 2024

Word History

First Known Use

1917, in the meaning defined above

Kids Definition

herd immunity

noun

: a reduction in the risk of catching a disease (as measles) that occurs when enough people have become immune to infection (as through previous exposure or vaccination) that susceptible people are far less likely to come in contact with infected people

Medical Definition

herd immunity

noun

ˈhərd- 

: 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