There have been signs that the virus established itself long before community transmission was recognized.
Researchers believe that cases in New York probably began circulating in mid-February. Trevor Bedford, a scientist at the Fred Hutchinson Cancer Research Center who studies the spread and evolution of viruses, has estimated that an outbreak in the Seattle area likely has roots back to about Feb. 1, or perhaps could be linked to the first reported case in the United States — a man who flew from China to the Seattle area on Jan. 15 and later tested positive.
But, until now, the first known deaths from the coronavirus were recorded in the Seattle area on Feb. 26.
Whether and how the early cases might be related to later cases remains unclear. C.D.C. scientists are attempting to discern the genetic sequences of the virus from the three cases, which can be challenging when tissues are obtained after death.
So far, only relatively short pieces of genetic material have been obtained, said Kristen Nordlund, a spokeswoman for the agency. That was enough to confirm the diagnosis, but not large enough to detect the specific virus’s subtype, which could help identify potential links to other cases.
Dr. Charles Chiu, who has been examining the genetic signatures of the viruses circulating in the Bay Area, had previous suspicions about the Santa Clara cases. While many of the viruses landing in the United States have several mutations compared with the original strain in Wuhan, one of the cases examined at the end of February in Santa Clara had only one mutation.
That indicated that the cases there had a strong link to the original Wuhan strain.
“It suggested that it was an early introduction,” Dr. Chiu said. Even still, he was surprised that a virus-related death occurred as early as it did in February. He is interested in seeing the genomic sequence of the new cases, although he suspects they are likely to be connected to the larger cluster in Santa Clara linked by the same signature mutation.
The virus has an incubation period of up to 14 days, and people can be infectious before symptoms begin. Because Ms. Dowd had been ill for several days before dying on Feb. 6, it is possible that she could have transmitted the infection to others as early as January.
“This offers evidence of what many of us in the field had been saying,” said Dr. Adalja, the infectious disease specialist. “That restricting testing was going to miss cases that could have a chain of transmission that ended up with somebody dying.”