The study found that some New Yorkers had antibodies to the virus as early as the week ending Feb. 23. Given the time needed to produce antibodies, those people were most likely infected with the virus about two weeks earlier.

“You’re probably talking about very early in February,” said Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai, who led the study. “It looks like there was at least low-level circulation.”…

Genetic analyses have suggested that the virus entered the city several times early in the year, but most of those introductions died out and did not initiate the city’s epidemic.

“If I had to put a single date on it, based on current models, we had it as Feb. 19 as the arrival that fueled things,” said Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle. Dr. Krammer’s date is only slightly earlier, he noted.