Results from New York state's first coronavirus antibody tests found that approximately 21 per cent of 1,300 New York City residents tested positive.
Governor Andrew Cuomo announced the findings at his daily press briefing on Thursday.
The percentage of positive test results in the city implies that approximately 1.6 million out of the 8.4 million people in the five boroughs have already been infected with the virus.
Mr Cuomo stressed the results were preliminary, but data will improve over time as more testing is done on a rolling basis.
Testing for antibodies was conducted over two days in 19 counties and 40 localities, at grocery stores and other large retailers.
A total of 3,000 people were tested statewide with an average positive result of 13.9 per cent across the state, implying 2.7 million people.
The antibody testing program is the second stage of the state's overall testing strategy. The first being the test, trace and isolate program for stopping the spread of the virus.
Mr Cuomo said that the infection rate would be a better indicator of how the state might go about reopening.
It could also allow those with antibodies to donate plasma for convalescent treatment.
The governor underlined the fact that those tested were out and about, shopping, not at work, and not in quarantine — which would be a factor to consider in the results.
There was a disparity between the male and female samples, with 12 percent of women and 15.9 per cent of men testing positive.
Regionally, New York City and Long Island has higher percentages of positive tests than the rest of the state as a result of population density.
In terms of race, the results for Black, Latino and multi-ethnic people tracked with the regional breakdown, their results mirroring New York City where the majority of people in those groups live.
The total figures for both the city and state are much higher than the officially confirmed cases. Anecdotal evidence suggested that this was going to be the case.
They also imply that the percentage death rate is much lower than thought, but as the official total number of deaths does not include people that may have died at home, that too is difficult to ascertain without more data.
As of Thursday, New York's official count of confirmed cases was 257,216 with 145,855 in New York City. Recorded deaths stood at 15,302 and 11,267 respectively.
