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

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

Those levels are well below what is considered adequate for herd immunity, which typically requires a vaccination rate of 95% to prevent the spread of highly contagious diseases. —Cindy Krischer Goodman, The Orlando Sentinel, 16 Apr. 2026

As the risk of measles remains an ongoing concern, herd immunity in Allegheny County, Pennsylvania, is already slipping. —Kar-Hai Chu, The Conversation, 10 Apr. 2026

Measles was all but eliminated and herd immunity achieved across multiple diseases. —Katie Silver, NPR, 28 Mar. 2026

Every crank needs a foil, just as every vaccine skeptic benefits from herd immunity. —Michael Tedder, The Atlantic, 15 Mar. 2026

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