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

The purpose of vaccination is to prevent populations from getting diseases through herd immunity, and not necessarily identifying at-risk individuals. —Omer Awan, Forbes.com, 18 Sep. 2025

Among the states collecting data for the measles-mumps-rubella (MMR) vaccine, 68% of counties and jurisdictions now have immunization rates below 95% — the level of herd immunity doctors say is needed to protect against an outbreak. —Jason Kane, NBC news, 15 Sep. 2025

Other physicians and health experts have long pointed to the benefits of herd immunity, or the idea that if enough people are vaccinated against a disease, then those who cannot be vaccinated because of age or certain medical conditions are also protected. —Mike Lillis, The Hill, 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

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