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 vaccination rates are high, an individual might benefit from herd immunity without accepting even the minimal vaccine risks. —Avi Dor, The Conversation, 5 June 2025

Researchers found that the county-level mean vaccination rate decreased from almost 94% before the pandemic to about 91% after the pandemic—falling well below what public-health experts generally consider to be the threshold for herd immunity against measles of about 95%. —Chantelle Lee, Time, 2 June 2025

This is because when more than 95% of people in a community are vaccinated, most are protected from measles through community immunity, also known as herd immunity, according to the CDC. MORE: Amid growing Texas outbreak, how contagious is measles? —Mary Kekatos, ABC News, 30 Apr. 2025

Parents could no longer reliably count on herd immunity to keep newborns too young to vaccinate safe from the disease. —Corinne Purtill, Los Angeles Times, 25 Apr. 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