herd immunity

American  

noun

1. the immunity or resistance to a particular infection that occurs in a group of people or animals when a very high percentage of individuals have been vaccinated or previously exposed to the infection.

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Usage

What is herd immunity? Herd immunity is immunity from or resistance to a specific infectious disease throughout a population that results from a majority of that population having been exposed to that infection, such as by having been vaccinated for it or having been previously infected by it. In medicine, the word immunity typically refers to the state of being unable to get a disease due to being protected from it by antibodies that will recognize it and immediately fight it if it enters the body. (Immunity isn’t always permanent—sometimes it wears off after months or years.) The word herd is used in the phrase to refer to a group of people or animals (the term was originally used in the context of cattle diseases). Herd immunity makes it less likely for an infected person to transmit the disease to others. That’s because the majority of the group (the herd) has become immune or strongly resistant to the disease by having antibodies that will fight it. These antibodies may have been produced from an earlier infection or through vaccination. Herd immunity relies on a large percentage (often 70-90%) of people being immune. When this is the case, infected people are unlikely to spread the disease because almost everyone they come in contact with is already immune or resistant to it. With herd immunity, even people who are not immune to the disease are less likely to be infected because the majority of people around them are immune and will not spread the disease to them. Medical experts most commonly discuss herd immunity in terms of the kind achieved through vaccination. In this scenario, vaccines must be effective, widely available, and widely used in order to eventually lead to widespread immunity. This method has led to the elimination or near elimination of diseases such as smallpox, polio, and chickenpox. Herd immunity can also occur when the majority of a population becomes infected. In this scenario, the immune systems of those who recover become able to recognize the disease and produce antibodies against it, preventing them from becoming reinfected. Of course, such a wide spread can result in many deaths. For this reason, many medical experts warn against pursuing such a scenario intentionally. Such a course of action was discussed and rejected by most epidemiologists during the COVID-19 pandemic in 2020, when the term herd immunity became more popularly known.

Example Sentences

“If the secretary were to say, under my judgment herd immunity is the best way, that would be unreviewable,” the lawyer said.

From Salon • Mar. 7, 2026

We no longer see devastating illnesses like polio and haemophilus meningitis in children, thanks to the effectiveness of herd immunity.

From MarketWatch • Mar. 5, 2026

But outbreaks happen when vaccination rates fall below the levels needed to maintain herd immunity, which is about 95%.

From The Wall Street Journal • Jan. 22, 2026

It explained that medical exemptions are far rarer and can be maintained without risking herd immunity.

From Slate • Dec. 9, 2025

California is also one of only 10 states with a kindergarten measles vaccination rate exceeding the 95% threshold experts say is necessary to achieve herd immunity.

From Los Angeles Times • Sep. 11, 2025