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

More recently, epidemiologists were debating what percentage of a population had to be vaccinated to reach herd immunity and when that threshold would be reached. —Larry Brilliant, Foreign Affairs, 8 June 2021

Independent monitors are now confirming the exact proportion of children who have received a dose of the campaign’s vaccine, but the initial round appears to have met the target required for herd immunity: a minimum of 90 percent of all children 10 years and younger. —Saima S. Iqbal, Scientific American, 26 Sep. 2024

Utah's vaccination rates fall far below the 95% needed for herd immunity against highly contagious illnesses like measles. —Erin Alberty, Axios, 17 Sep. 2024

Health officials said 95% immunization is necessary for a population to reach herd immunity and stop the spread of measles. —Isabel Funk, USA TODAY, 4 Sep. 2024

Word History

First Known Use

1917, in the meaning defined above

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