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

Vaccination rates have declined in recent years, however, resulting in a growing number of states no longer reporting rates consistent with herd immunity, and infections have returned. —Mary Walrath-Holdridge, USA Today, 22 Jan. 2026

Disease was even prevented among unvaccinated kids, older children, and adults — the demonstrable benefit of herd immunity. —Ben Lopman, STAT, 17 Jan. 2026

This map of per-county kindergartner vaccination percentages in Utah and Arizona shows that all but two counties have percentages below the 95% level needed for herd immunity, with nine counties having percentages lower than 85%. —Erika Edwards, NBC news, 16 Jan. 2026

High vaccination rates build herd immunity, which helps prevent disease transmission to high-risk groups including newborns, the elderly and cancer patients undergoing chemotherapy. —Allison Ong, Sun Sentinel, 12 Jan. 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