Here in New York City, the death toll from COVID-19 is nearing 16,400, according to data compiled by Johns Hopkins University. On Thursday, Governor Andrew Cuomo said about one in five New York City residents has antibodies to the novel coronavirus in their blood, suggesting infection rates are an order of magnitude higher than the number of confirmed cases. The announcement was based on preliminary results from a study that randomly sampled 3,000 people at grocery stores across New York state.

If the numbers hold, the death rate from coronavirus infections in New York is about 0.5% — less lethal than World Health Organization estimates but still many times deadlier than seasonal flu. And the novel coronavirus remains far more contagious than influenza.

Health experts are concerned about the accuracy of dozens of new serology tests that have only just reached the market, with little vetting by the Food and Drug Administration. And it’s still not known whether people with antibodies to coronavirus are immune to a second round of the disease — or how long any immunity might last.