Researchers Estimate There Were 17 Million Undiagnosed Cases Of Covid-19 Last Summer

A new study from the National Institutes of Health found that almost 5% of Americans surveyed had undiagnosed Covid-19 last summer — which amounts to almost 17 million cases that were not counted. This finding, which is currently undergoing peer review ahead of journal publication, suggests the pandemic was much more widespread in the U.S. than previously thought.

For most respiratory viruses, “the true extent of the spread is really not known until after the outbreaks occur,” says study senior author Kaitlyn Sadtler, an immunologist at the NIH and Forbes 30 Under 30 alumna. “It is after those waves that we really learn the damage that was done in terms of case numbers.”

The study surveyed more than 11,000 Americans who had not previously been diagnosed with Covid-19. The researchers took blood samples from volunteers between May and July 2020 and analyzed the blood samples for antibodies against Covid-19. They found that 4.6% of the study participants had antibodies to Covid-19 even though they had never been diagnosed with the illness, which amounts to 4.8 undiagnosed cases of Covid-19 for every diagnosed case over the summer of 2020— or almost 17 million cases that went undetected by mid-July 2020. 

Though official counts vary, there have been about 26 million cases of COvid-19 reported in the U.S. since the beginning of the pandemic. But for months, experts have warned that the actual number of cases is much higher. “[The study] gives us a window into the beginning of the pandemic,” Sadtler says. She says that while some of these volunteers reported being sick at some point during 2020, many did not report any illness at all. 

Some researchers estimate that about 30-40% of Covid-19 cases are asymptomatic, meaning that people don’t have symptoms. This isn’t unusual – other illnesses such as influenza or polio also have high percentages of asymptomatic cases.This has made the pandemic more difficult to contain because even asymptomatic people can be highly contagious. A recent study published in JAMA found that almost 60% of all Covid-19 transmission came from people who were asymptomatic. 

 Sadtler says that she hopes other research groups can use the data in this study to understand more about asymptomatic transmission. As for her group, they’ll be following the same volunteers for the next year and taking blood samples twice more. That way, she says, “we can look at the evolution of the antibody responses of these individuals out in the public,” and figure out if and when their antibodies disappear, as well as if new participants get infected with Covid-19. 

Like this current paper, the next phase of Sadtler’s research will be focused on data that will be months in the past by the time it is published. Which raises a question: how can researchers and health officials get an accurate picture of how many people are infected with Covid-19 right now? We need to keep doing the things that we know work, Sadtler says, including contact tracing and increased testing. “It’s just investing the time and effort.”