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

This number should be zero or close to it if herd immunity existed, which refers to the protection conferred onto a community against a contagious disease when a certain percentage of the population becomes immune. —Omer Awan, Forbes, 22 Dec. 2024

Reducing the vaccines required for schooling or insurance coverage could not only put individuals at risk, but also reduce herd immunity and increase disease outbreaks in schools and communities. —Alyssa Goldberg, USA TODAY, 14 Nov. 2024

Smallpox was endemic among the British giving them a natural herd immunity to the illness. —John Laycock, Baltimore Sun, 27 Dec. 2024

The decline in childhood vaccination rates threaten herd immunity and will ultimately allow diseases that were thought to be eradicated to reappear. —Omer Awan, Forbes, 15 Dec. 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