herd immunity
Americannoun
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
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
Public-health leaders expect measles to keep spreading until they reach herd immunity, meaning some 95% of people have either contracted the virus or been vaccinated.
From The Wall Street Journal ● Oct. 7, 2026
A 95 percent vaccination rate with the modern, two-dose MMR shot—so named because it also protects against mumps and rubella—is enough to provide herd immunity from measles.
From Slate ● Jul. 24, 2026
The United States has also seen a rise in measles cases in recent years, and children under five in both countries fall short of the 95% vaccination threshold needed for herd immunity.
From BBC ● Jul. 8, 2026
The spike in cases comes as vaccination rates have tumbled nationwide in recent years — in some areas falling well below the herd immunity threshold experts say is necessary to keep it from spreading.
From Los Angeles Times ● Jul. 8, 2026
“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
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