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

What Florida would be choosing School vaccination requirements help communities establish herd immunity, which is when enough people are vaccinated to prevent a disease from easily spreading. — Katrine L. Wallace, The Conversation, 30 Oct. 2026

When less than 95% of a community is vaccinated for measles, herd immunity is lost, meaning there aren’t enough immune people to prevent the spread of infection. — Katie Lockwood, STAT, 28 Oct. 2026

For measles, the vaccination threshold for herd immunity is 95%. — Brittney Melton, NPR, 13 Oct. 2026

For measles, populations need vaccination rates of 95 percent or higher to achieve herd immunity—the level of protection required to prevent disease spread. — Lauren J. Young, Scientific American, 18 Sep. 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