Reported coronavirus cases vastly underestimate the true number of infections, U.S. government data suggest, echoing results from a smaller study last month.
Two data sets from the Centers for Disease Control and Prevention published Tuesday - one in the journal JAMA Internal Medicine and another on the agency's website, based on follow-up data - say true COVID-19 rates are more than 10 times higher than reported cases in some U.S. regions.
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The estimates are based on COVID-19 antibody tests performed on routine blood samples in 16,000 people in 10 U.S. regions.
The researchers likely detected infections in people who may have had no symptoms or only mild illness, and who never got coronavirus tests.
In the first set of data, taken from late March to early May, estimated infection rates were from six times higher than reported cases in Connecticut to 24 times higher in Missouri.
The second set of data, which included a later round of antibody testing at eight of the 10 sites included in the original study, estimates that infections are between two and 13 times higher than the current case counts in the U.S., lead study author Dr. Fiona Havers, a member of the CDC seroprevalence task force, said. The ones without updates were Louisiana, with an infection rate of nearly 6 percent of the state's population, and San Francisco, with an infection rate of 1 percent.
In Connecticut, the estimated infection rate hovered around 5 percent in both data sets. In Missouri, it hovered under 3 percent.
While cases may be higher than reported, the estimates indicate that the vast majority of Americans have not been infected, and therefore remain vulnerable.
"Even in hard hit areas like New York City, the majority of people have not yet been infected with the virus," she said. With the second set of data, the CDC estimates that 23 percent of the city's population have antibodies.
What's more, it's still unclear whether antibodies confer immunity, and if so, how long that immunity lasts, Havers said. "We don't know if antibodies represent protection from the virus. And there may be some suggestion that antibodies waning over time, so we don't know if herd immunity is achievable," she said.
The most important thing to takeaway from the estimates, Havers said, is that the public needs to double down on social distancing measures recommended by public health officials.
"Many people may not realize that they're infected, but they could still be transmitting the virus," Havers said. "It's critical that the public follows the public health recommendations like wearing cloth face coverings, remaining 6 feet apart from other people, washing hands frequently, and staying home when sick."