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

By requiring immunization before school enrollment, society created a collective shield — herd immunity — that protects even those who cannot be vaccinated due to medical reasons. —A.j. Russo, Baltimore Sun, 5 Mar. 2026

In particular, babies younger than 12 months of age and people who are immunocompromised or have a medical condition that prevents them from receiving the vaccine benefit from high herd immunity, Orenstein explains. —Lauren J. Young, Scientific American, 27 Feb. 2026

Without herd immunity to protect him, a 20-year-old unvaccinated man became paralyzed. —Jon Lapook, CBS News, 22 Feb. 2026

Some estimates suggest 80% of healthy people and 90% of high-risk people need to be vaccinated for herd immunity against the flu, though these thresholds can be difficult to achieve, Huang said. —William Tong, Dallas Morning News, 29 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