“New research indicates that the coronavirus began to circulate in the New York area by mid-February, weeks before the first confirmed case, and that travelers brought in the virus mainly from Europe, not Asia.
“The majority is clearly European,” said Harm van Bakel, a geneticist at Icahn School of Medicine at Mount Sinai, who co-wrote a study awaiting peer review.
A separate team at N.Y.U. Grossman School of Medicine came to strikingly similar conclusions, despite studying a different group of cases. Both teams analyzed genomes from coronaviruses taken from New Yorkers starting in mid-March.
The research revealed a previously hidden spread of the virus that might have been detected if aggressive testing programs had been put in place.
On Jan. 31, President Trump barred foreign nationals from entering the country if they had been in China during the prior two weeks.
It would not be until late February that Italy would begin locking down towns and cities, and March 11 when Mr. Trump said he would block travelers from most European countries. But New Yorkers had already been traveling home with the virus.
“People were just oblivious,” said Adriana Heguy, a member of the N.Y.U. team.