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New study suggests many were infected last year but never diagnosed with COVID-19

A coronavirus vaccination being prepared. Even if you've had the coronavirus, you should still get your shot, public health experts say.Lane Turner/Globe Staff

A recently released study offers new clues to an unanswered question about the coronavirus pandemic: How many people have gotten the virus but were never officially diagnosed?

National Institutes of Health researchers reported this week that the prevalence of COVID-19 in the United States during spring and summer of last year was much higher than the known number of cases.

The researchers, in a paper published in Science Translational Medicine, estimated “there were approximately 4.8 undiagnosed infections for every identified case of COVID-19, suggesting a potential 16.8 million undiagnosed infections by July 2020.”

“Our data suggest a larger spread of the COVD-19 [sic] pandemic in the United States during the first six months than originally thought,” the study said.


The study did not address how many people might have undiagnosed infections now. The US total of diagnosed infections has grown to around 33.4 million on Thursday.

“A hallmark of the coronavirus pandemic is that there are people infected with the virus that causes COVID-19 who have few or no symptoms,” said Dr. Matthew J. Memoli of the National Institute of Allergy and Infectious Diseases, one of several NIH institutes leading the study.

“While counting the numbers of symptomatic people in the United States is essential to contend with the impact of the pandemic and public health response, gaining a full appreciation of the COVID-19 prevalence requires counting the people who are undiagnosed,” he said this week in an NIH statement.

Researchers came up with their estimates after looking at blood samples from a representative sample of over 8,000 undiagnosed people, searching for antibodies against the SARS-CoV-2 virus, which causes COVID-19, the NIH said.

The findings come as the United States is struggling to get as many people as possible vaccinated against the virus, while officials warn of the worrisome new delta variant.


Other studies have also suggested that large numbers of cases have gone undiagnosed. A Columbia University researcher estimated earlier this year, based on computer modeling, that the number of undiagnosed people continued to be multiple times higher than the number of diagnosed people.

Dr. Thomas Tsai, a professor at the Harvard T.H. Chan School of Public Health, said the NIH study “suggests that there may be a higher than expected level of infection-induced immunity currently in the US population,” but he said he wasn’t aware of any definitive estimate of that level.

It may sound reassuring that, in addition to the 177 million people who have gotten at least one dose of vaccine, there are many millions who may have some natural protection against the coronavirus after contracting it and never getting tested for it.

But experts emphasize people should get their shots even if they’ve had COVID-19.

“You should be vaccinated regardless of whether you already had COVID-19,” the CDC says on its website. “That’s because experts do not yet know how long you are protected from getting sick again after recovering from COVID-19. Even if you have already recovered from COVID-19, it is possible—although rare—that you could be infected with the virus that causes COVID-19 again. Studies have shown that vaccination provides a strong boost in protection in people who have recovered from COVID-19.”

Tsai said, “I think getting vaccination is still incredibly important. Especially with the delta variant which has been shown to have increased household transmission and increased severity of illness, vaccination remains important to protecting the health of individuals and their family members.”


Jeremiah Manion of the Globe staff contributed to this report.

Martin Finucane can be reached at martin.finucane@globe.com.