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

Emphasize the importance of community health and herd immunity. —Kristen Moon, Forbes.com, 10 Sep. 2025

In many areas, vaccination rates have fallen below the 95% threshold in kindergarteners needed to create herd immunity against measles. —Brenda Goodman, CNN Money, 9 Sep. 2025

Those children often rely on herd immunity, which is reached when nearly an entire population is vaccinated against a disease, to be safe in the classroom. —Sarah Fortinsky, The Hill, 7 Sep. 2025

This is most effectively done by achieving what is known as herd immunity through vaccinations. —Mary Walrath-Holdridge, USA Today, 5 Sep. 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