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

But vaccinations have declined, resulting in a growing number of states no longer reporting rates consistent with herd immunity, and infections have returned. —Mary Walrath-Holdridge, USA Today, 19 Aug. 2025

But when the vaccine efficacy is 60% the vaccination coverage needed to achieve enough herd immunity to stop the spread of the virus goes up to 83%. —Bruce Y. Lee, Forbes.com, 16 Aug. 2025

Schools that achieve herd immunity reduce the risk of outbreaks, according to DPH. —Livi Stanford, Hartford Courant, 6 Aug. 2025

In 2000, the Centers for Disease Control and Prevention declared that measles had been effectively eliminated in the U.S., thanks to vaccine rates that hovered around 95%, the level needed for herd immunity. —Michael R. Bloomberg, Mercury News, 25 July 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