What if the virus quietly arrived in December?

Doctors in France have said that a patient’s sample from late December has since tested positive for the coronavirus. But so far, there is no comparable evidence of a similar case in the United States.

The strongest possible indicator so far is new evidence that emerged this week of two people in Snohomish County, Wash., who reported coronavirus-like symptoms in December. Both people later tested positive for antibodies, county health officials announced.

But Dr. Chris Spitters, the county’s health officer, said that while it was possible that both people had the coronavirus in December — even before officials in China had reported a cluster to the World Health Organization at the end of the month — he was doubtful.

“It’s possible and frankly, I think, more likely that they had a non-Covid respiratory viral illness in December and subsequently had an asymptomatic or minimally symptomatic Covid infection subsequent to that,” Dr. Spitters said.