More than 21 percent of roughly 1,300 New York City residents tested as part of a state study had antibodies linked to COVID-19, Governor Andrew Cuomo revealed Thursday during a press conference.

According to the preliminary results of the survey, 21.2 percent of those tested in NYC had antibodies, compared with 16.7 percent on Long Island, and 11.7 percent in Westchester and Rockland counties. In the rest of the state, just 3.6 percent of those tested had been infected and recovered from the disease.

Statewide, nearly 14 percent in the sample of 3,000 people tested positive for the antibodies.

Extrapolating these infection rates to the broader population, that would mean that more than 2.6 million people in the state and more than 1.7 million people in New York City have already been infected with coronavirus.

The state only tested people who were visiting supermarkets and big box stores. In doing so, the study rules out those who were isolating at home.

The tests confirmed the disparities that have been observed so far in those who have suffered most from the disease. Testing showed that more than 22 percent of black and Latino New Yorkers have had the virus, compared with about 9 percent of whites.

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[Listen to WNYC's Fred Mogul talk about the results of the state's antibody study.]

It’s still unclear what level of antibodies are needed in order for someone to be immune to the novel coronavirus, and there can be false positives. But widespread antibody testing is still crucial, Cuomo said.

If the results are accurate, the tests can identify who is able to donate blood to be used to help treat those who are currently infected. Widespread testing can also provide a clearer picture of the spread of the disease.

“This basically quantifies what we’ve been seeing anecdotally and what we have known but it puts numbers to it,” Cuomo said.

He also argued that it could support different policies in different parts of the state.

“What you do in a place with 21 percent [infection rate] is not what you do in a place with 3.6 percent,” he said.

The new data also revealed that the estimated death rate from the disease might be lower than some estimates show, although Cuomo expressed some doubt about those outcomes.

With some 15,500 deaths from coronavirus recorded in New York state so far, that places the death rate at 0.5 percent.

“But that 15,500 is not the accurate total number of deaths,” Cuomo said. “It does not count in-home deaths.”

He also noted that people in New York may have been dying of coronavirus before the state started tracking the disease.

California Governor Gavin Newsom has ordered a review of autopsies dating back to December in order to better understand the spread of the disease. It’s unclear whether Cuomo will follow suit.

Antibody testing will continue on a rolling basis to provide updated snapshots of how the disease is progressing, Cuomo said. He cautioned that the results were preliminary.

On Wednesday, the city’s infectious disease specialist, Dr. Demetre Daskalakis, sent an email alert to healthcare providers warning that the antibody tests were unreliable and that they should not be used to determine "durable immunity."

"They may produce false negative or false positive results, the consequences of which include providing patients incorrect guidance on preventive interventions like physical distancing or protective equipment," he wrote.

Other experts criticized the state for not providing its official methodology on testing.

"I think it’s worth putting a little more detail about your plan out there, so people can scrutinize it," said Dr. Denis Nash, an epidemiologist who teaches at the Hunter College School of Public Health and previously conducted a similar antibody survey for West Nile Virus while working at the City Health Department. "We should at least have some transparency about it. I think it is irresponsible to report these kinds of estimates without detailing the methodology and giving information about the uncertainty of all the estimates."

Dr. Stephen Morse, an epidemiologist at the Columbia Mailman School of Public Health questioned the sample size.

"Something is better than nothing," he said, "but of course that's not nearly enough to estimate seroprevalence for the entire state."

Fred Mogul contributed reporting to this story.