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

Risk of vaccination rate decline Nationwide, at least 10 states have seen vaccination rates fall, jeopardizing the country's herd immunity from widespread outbreaks. —Sara Moniuszko, CBS News, 12 Feb. 2025

Why Polio Could Still Enter the U.S. A population’s protection against polio outbreaks is largely influenced by herd immunity—the threshold at which a large enough proportion of people are immune to a disease from vaccination or infection. —Tara Haelle, Scientific American, 30 Jan. 2025

The oral vaccine, which was discontinued in the U.S. in 2000, is more effective at achieving herd immunity because people shed the virus in their feces and saliva, and those who don’t receive the vaccine may come in contact with the weakened viruses and develop antibodies against polio. —Tara Haelle, Scientific American, 30 Jan. 2025

Achieving herd immunity to limit the spread of disease depends on collective effort, but many see it as a personal choice. —Barbara A. Perry, Newsweek, 27 Jan. 2025

Word History

First Known Use

1917, in the meaning defined above

Rhymes for herd immunity

• autoimmunity
• importunity
• opportunity
• community
• disunity
• impunity
• munity
• unity

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