The term has been used a lot since the pandemic began—here's what it means.

By Claire Gillespie
Updated March 18, 2020

With COVID-19 now a global pandemic, and cases of the new coronavirus confirmed in 46 states, the big question is, what happens next?

Following a call from the World Health Organization for countries to “take urgent and aggressive action,” world leaders are holding crisis talks with health officials to figure out the best way to protect the public from the coronavirus that’s caused more than 8,000 deaths worldwide.

One of the things being talked about is herd immunity, but what does this mean, and how does it relate to COVID-19?

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Herd immunity (also known as community immunity) is defined by the Centers for Disease Control and Prevention (CDC) as “a situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely.”

In other words, where herd immunity exists—when lots of people in an area are vaccinated or have already been infected with a disease—fewer people get sick, and fewer germs are able to spread from person to person. 

The CDC adds that even people who are not vaccinated, like newborn babies and individuals with chronic illnesses, have some level of protection because the disease can’t spread within the community. 

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The theory behind herd immunity is that when someone gets vaccinated, it’s not only that person who is protected from infection—they can’t transmit the disease to other people. Herd immunity protects people who cannot be vaccinated because their immune systems aren’t strong enough and are therefore the most vulnerable to serious illness. 

An example of herd immunity via vaccination is the measles outbreak among preschool-age children in the US in the late 1980s. The attack rate decreased faster than coverage increased, and researchers who examined the association between incidence of measles and immunization coverage among preschool-age children concluded that immunization coverage of about 80% may be enough to stop sustained measles outbreaks in an urban community.

Of course, there’s no vaccine for COVID-19 yet. So the herd immunity situation is a little different. The only option is recovery, which means letting the majority of people catch the virus at some point. 

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President Trump hasn’t mentioned this approach. But in the UK, which had nearly 2,000 confirmed infections as of March 18, per Johns Hopkins University's global coronavirus tracking system, Prime Minister Boris Johnson had reportedly been considering herd immunity as a strategy. However, on March 17, BBC reported that the UK had shifted gears based on new modeling on the severity of the situation and the number of people who might die. 

Absent a vaccine to protect people against COVID-19, waiting for herd immunity to occur "is not a good public health strategy," writes UK virologist Jeremy Rossman, PhD. Given the rapid rate of infections around the globe, public health officials in the US are looking to contain the spread of the virus by limiting person-to-person contact and urging the public to take steps to protect themselves and others.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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