(Bloomberg) --

A New York state study seeking to find out how many people have been infected by the new coronavirus found that 13.9% of those tested across the state had signs of the virus, in one of the biggest U.S. reviews to date.

In New York City, the hardest-hit area in the U.S., 21.2% of people tested positive for a blood marker showing that they had been infected at some point. Statewide, 2.7 million people 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 of mostly very sick patients.

There are broad ramifications of the study, which will need to be analyzed further by experts to give a clearer picture of the infection’s prevalence. But it appears to confirm predictions that the virus has infected far more people than New York has been able to diagnose so far.

It also means that the fatality rate of the virus is likely lower than the figure that’s based only on confirmed cases and deaths. Officially, New York has reported 15,500 deaths. If 2.7 million people have been infected, that would put the fatality rate at around 0.6%.

The current death count doesn’t include some people may have died at home and not been diagnosed, 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 exposed. They can show whether a person was previously exposed to 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.

According to the survey, New York City had the highest rate of positive tests, followed by Westchester and Rockland counties with 11.7% positive and, Long Island with 16.7% positive, and the rest of the state with 3.6% positive.

About 3,000 samples were collected from 19 counties, from people shopping at grocery and big box stores, Cuomo said at a briefing outlining the results. They may not be perfectly representative of the illness, he said, because of who was sampled.

“This basically quantifies what we have been seeing anecdotally,” Cuomo said.

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