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

That sounds high but measles is so contagious that a vaccination rate of at least 95% is required for herd immunity. —Amy Feldman, Forbes, 12 Mar. 2025

Vaccination rates at more kindergartens have been dipping below the 95% level previously recommended (and in California, currently enforced) for herd immunity. —Jia H. Jung, The Mercury News, 11 Mar. 2025

The declaration called for speeding herd immunity by allowing people at low risk to get infected while protecting those most vulnerable, such as the elderly. —Rob Stein, NPR, 5 Mar. 2025

But nationwide, at least 10 states have seen childhood vaccination rates decline, jeopardizing communities' herd immunity from widespread outbreaks. —Sara Moniuszko, CBS News, 3 Mar. 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