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 World Health Organization says there needs to be at least 95% of vaccination in a population for herd immunity. —Michelle Marchante, Miami Herald, 27 Feb. 2024

That is below the 95% vaccination threshold health experts say is needed for herd immunity against measles. —Michelle Marchante, Miami Herald, 22 Feb. 2024

The World Health Organization considers 95% the threshold necessary for herd immunity from measles. —Katie Mogg, NBC News, 20 Feb. 2024

Last school year, that dropped to 92.8% — below the 95% needed for herd immunity. —Stephen Stock, Leigh Ann Winick, CBS News, 23 Jan. 2024

Another state, Louisiana, joined the suit along with three doctors who co-signed a paper on herd immunity, an anti-lockdown activist in Louisiana, and a conservative news site, The Gateway Pundit. —Sarah Owermohle, STAT, 6 Feb. 2024

The share of schools below herd immunity has fallen from 50% to 20%, and exemptions have fallen to less than 2%. —Stephen Stock, Aparna Zalani, Chris Hacker, CBS News, 30 Jan. 2024

The percentage of a community that needs to be immune to achieve herd immunity varies from disease to disease. —Karen Pallarito, Health, 20 Jan. 2024

Fast forward two years and the brand loyalty has faded, herd immunity has taken hold, and staying up to date on your vaccines can be confusing. —Rachel Murphy, Verywell Health, 30 Nov. 2023

Word History

First Known Use

1917, in the meaning defined above

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