A New York study seeking to measure the spread of the new coronavirus found that 13.9% of 3,000 people tested across the state had signs of the virus, one of the biggest U.S. reviews to date.

That implies that about 2.7 million residents may have had Covid-19, Governor Andrew Cuomo said. That’s about 10 times more than the official count based on the state’s testing, which covered mostly very sick patients.

The pandemic was more intense in New York City, the hardest-hit area in the U.S. There, 21.2% of people tested positive for a blood marker showing that they had been infected at some point.

“We’re going to continue this testing on a rolling basis,” Cuomo said. “We’ll have a larger and larger sample. But I want to see snapshots of what is happening with that rate. Is it going up? Is it flat? Is it going down? And it can really give us data to make decisions.”

The study must be analyzed further to give a clearer picture, and it’s possible that it didn’t give a representative sample because it likely missed older people staying home as well as essential workers, Cuomo said. Still, the preliminary results appear to confirm predictions that the virus has infected far more people than New York has been able to diagnose.

It also means that the fatality rate is likely lower than implied by merely examining confirmed cases and deaths. If 2.7 million people have been infected, that would put the fatality rate at about 0.5%, based on the death count of 15,500 the state used to make its calculation. Since then, the number of deaths in New York has risen to 15,740.

The current death count doesn’t include some people who may have died at home and weren’t diagnosed with Covid-19, and may also miss people who died earlier on in the outbreak before diagnostic testing became widespread.

The survey used blood tests that look for antibodies, which are markers in the blood created by the immune system after a person has been infected. They can show whether a person was previously infected by the virus, even after they have recovered from the illness. Antibody tests may also help find people who were infected but showed few or no symptoms.

Diagnostic tests, such as the nasal swabs used on people who show up sick at the hospital, check for the virus itself. They can’t find out whether people were previously infected and recovered, however.

The antibody testing took samples from 40 locations in 19 counties, from people over the age of 18 who were shopping at grocery and big-box stores, Cuomo said.

The weighted results showed more than 1-in-5 New Yorkers testing positive, as well as 16.7% of those in Long Island and 11.7% of those in Westchester and Rockland counties. The estimates showed 3.6% testing positive in the rest of the state.

Positive tests were highest among multiracial people -- at 22.8% -- along with African-Americans and Latinos, both around 22%.

New York has been focused on increasing testing for the coronavirus and antibodies. The federal government has agreed to help double capacity to 40,000 tests per day.

The number of new cases in the state remained relatively flat during the 24 hours that ended at midnight Wednesday, with 6,244 reported for a total of 263,460 positive cases statewide. Hospitalizations declined to 15,021 people. There were 438 deaths.

“That number is not coming down as fast as we would like to see that number come down,” Cuomo said.

With diagnostic testing across the country hamstrung by logistical failures, antibody tests could play a crucial role in helping officials determine how widespread the virus actually is, and therefore how to prevent more deaths.

“Estimates have been that in New York, the epicenter of the U.S. infection, the rate could be between 10 to 15%,” said Eric Topol, director of the Scripps Research Translational Institute. “This is in keeping with that.”

Uncertainty surrounds the accuracy of antibody tests, he said. The New York study could include false positives, he said, but by randomly sampling people, it avoided problems that plagued other surveys in which participants volunteered or were selected.

Topol said many people likely never knew they were carriers.

“It’s very high,” he said, “But obviously still way below what you would consider herd immunity.”

Many other such studies are under way. So far, early results indicate that many people who contract Covid-19 don’t show symptoms, but those numbers also vary widely from place to place.

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