The findings confirm suspicions raised in Santa Clara County last month that the virus had been circulating there since January, based on the discovery of two early victims of COVID-19 whose deaths previously had not been tied to the virus.
The first reported coronavirus cases in the United States were in late January, among travelers in several states. They had recently returned from China, which at the time was the epicenter of the pandemic. The first cases of community transmission — the virus spreading undetected among local residents — were identified in the end of February, in the Bay Area.
But evidence increasingly has suggested that the virus was spreading much earlier. Friday’s paper released by the Centers for Disease Control and Prevention looked at flu surveillance reports, retrospective coronavirus testing of several thousand respiratory samples, genomic testing, and the early deaths discovered in Santa Clara County.
Taken together, the CDC now believes that community spread began in late January or early February, possibly from a single imported infection from China. The report suggests that a man in Washington state — at the time the first coronavirus patient identified in the U.S., on Jan. 20 — may have been the source of the initial spread of disease there.
This story will update.
Erin Allday is a San Francisco Chronicle staff writer. Email: [email protected] @ErinAllday