16.8M COVID-19 cases went undiagnosed last summer, NIH study finds

COVID-19 surges: Dr. Fauci says everyone needs to get vaccinated as soon as possible

Dr. Anthony Fauci warns during a White House COVID-19 Task Force meeting that those who don't get vaccinated may risk localized surges in their communities.

A new study revealed that nearly 17 million COVID-19 cases went undiagnosed last spring and summer, illustrating how quickly the virus can spread, particularly among asymptomatic patients.

The National Institutes of Health released its findings Tuesday, which said that for every diagnosed COVID-19 case, there are 4.8 undiagnosed cases — estimating 16.8 million undiagnosed cases occurred by July of 2020 alone.

Scientists tested more than 8,000 people who hadn’t been previously diagnosed with COVID-19. Each participant gave a blood sample between May 10 and July 31, 2020. Scientists looked to find any antibodies that developed from a COVID-19 infection.

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Researchers found that more than 300 participants had COVID-19 antibodies, signaling they once had the virus. Researchers believed that sample meant 4.6% of people in the U.S. had an undiagnosed COVID-19 infection.

"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," Dr. Matthew J. Memoli of National Institute of Allergy and Infectious Diseases said.

During the study, scientists observed most of the undiagnosed cases were among women between 18 and 44 years old. They also found many undiagnosed cases among Black participants living in urban areas along the East Coast. The study didn’t provide a reason as to why undiagnosed cases were higher for a particular demographic.

"This study helps account for how quickly the virus spread to all corners of the country and the globe," Dr. Bruce Tromberg of the NIH said. "The information will be invaluable as we assess the best public health measures needed to keep people safe, as new—and even more transmissible—variants emerge and vaccine antibody response changes over time."

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Researchers will continue and evaluate the participants.

According to the U.S. Centers for Disease Control and Prevention, asymptomatic cases are people infected with COVID-19 but who do not exhibit any of the symptoms. Officials said it’s hard to track those numbers because asymptomatic people aren’t usually tested for COVID-19 unless they are required to or unless they take part in a scientific study.

Some studies suggest that even asymptomatic COVID-19 patients are at risk for medical problems. The American College of Cardiology featured a study revealing young asymptomatic COVID-19 patients could still develop heart inflammation from the virus.

Health officials also believe that asymptomatic patients can still spread the COVID-19 virus to others.

This story was reported from Los Angeles.