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

When more than 95% of the people in a community are vaccinated, most people are protected through community immunity, often referred to as herd immunity, according to the U.S. Centers for Disease Control. —Robert A. Cronkleton, Kansas City Star, 28 Mar. 2025

The document — which came before the availability of COVID-19 vaccines and during the first Trump administration — also promoted herd immunity. —CBS News, 25 Mar. 2025

The current measles outbreaks demonstrate the importance of herd immunity—a vaccinated person is highly affected by the vaccination status of those around them. —Ripley Cleghorn, Scientific American, 20 Mar. 2025

Ultimately, both Kennedy and much of unvaccinated America are receiving a crash course on the topic of herd immunity. —Dr. Jerome Adams, CNN, 20 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