herd immunity


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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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.

Why is herd immunity important?

The term herd immunity has been used since at least the late 1800s. Its early uses referred to actual herds of cattle. By the 1930s, the term was also being applied to human populations.

The more contagious a disease is, the higher percentage of immunity is needed to stop its spread. Herd immunity often requires an especially large percentage of the population to be immune in order to reasonably consider the people who are not immune to be protected. For example, it is estimated that herd immunity from measles, which is very contagious, requires up to 95% of the population to have immunity in order for the people who aren’t immune to be considered reasonably protected from the disease. If such a percentage isn’t maintained, the disease is much more likely to spread throughout the population.

Did you know ... ?

Medical experts have estimated that at least 70 percent of a population would need to be immune to COVID-19 to achieve herd immunity. In the United States, this would be nearly 230 million people. Globally, this would be nearly 5.5 billion people.

What are real-life examples of herd immunity?

Historically, the term herd immunity has mostly been used by medical experts, but it became more popularly known during the COVID-19 pandemic.



What other words are related to herd immunity?

Quiz yourself!

True or False?

For herd immunity, only a small percentage of a population needs to have immunity to a disease.

Medical definitions for herd immunity

herd immunity
[ hûrd ]


Resistance to the spread of infectious disease in a group because susceptible members are few, making transmission from an infected member unlikely.
The immunologic status of a population, determined by the ratio of resistant to susceptible members and their distribution.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.