As many as 442,000 Los Angeles County residents might have already been infected with the coronavirus by early April, a number far higher than the 8,000 cases confirmed at the time, according to a report released Monday.

In the first large-scale study of adults tested for COVID-19 antibodies in their blood, initial results show researchers found that 4.1% have the antibodies, an indication that they have already been exposed to the virus.

“That translates to roughly 221,000 to 442,000 adults who have recovered from an infection, once margin of error is taken into account, according to the researchers conducting the study,” the Los Angeles Times reported. “The estimated infection numbers are 28 to 55 times higher than the 7,994 confirmed COVID-19 cases L.A. County had reported at the time of the study in early April.”

The study, conducted by the University of Southern California and the Los Angeles County Department of Public Health, tested more than 860 people deemed representative of L.A. County, the researchers told The Times.

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“We haven’t known the true extent of COVID-19 infections in our community because we have only tested people with symptoms, and the availability of tests has been limited,” said lead investigator Neeraj Sood, professor of public policy at the USC Price School for Public Policy. “The estimates also suggest that we might have to recalibrate disease prediction models and rethink public health strategies.”

Said The Times: “The findings suggest the fatality rate may be much lower than previously thought. But although the virus may be more widespread, the infection rate still falls far short of herd immunity that, absent a vaccine, would be key to return to normal life.”

The Food and Drug Administration (FDA) earlier this month authorized the first blood test, known as a serology test, to look for antibodies in the blood. Cellex Inc., a medical device company based in North Carolina, says the test could help physicians determine how widespread the virus is and the duration of immunity for people after they recover.

Other new reports also indicate many Americans have already had the coronavirus but showed no symptoms at all.

“Based on known cases, health officials have said the virus usually causes mild or moderate flu-like illness,” The Associated Press reported Monday. “Now evidence is growing that a substantial number of people may have no symptoms at all.”

In the last week, reports of silent infections have come from a homeless shelter in Boston, a U.S. Navy aircraft carrier, pregnant women at a New York hospital, several European countries and California. The head of the U.S. Centers for Disease Control and Prevention says 25% of infected people might not have symptoms. The vice chairman of the Joint Chiefs of Staff, Gen. John Hyten, thinks it may be as high as 60% to 70% among military personnel. None of these numbers can be fully trusted because they’re based on flawed and inadequate testing, said Dr. Michael Mina of Harvard’s School of Public Health. Collectively, though, they suggest “we have just been off the mark by huge, huge numbers” for estimating total infections, he said.

In a random study of 200 residents on a single street in Chelsea, Massachusetts, about a third of participants tested positive for coronavirus, according to researchers.

That study, too, was not scientific, but “still, it’s kind of sobering that 30 percent of a random group of 200 people that are showing no symptoms are, in fact, infected,” Thomas Ambrosino, Chelsea’s city manager, told the Boston Globe.