How early did local transmission of the SARS-CoV-2 virus begin in the United States? For the second time [recently], scientists have proposed a new estimate. This one, from scientists at the Centers for Disease Control and Prevention, suggests that transmission likely began in late January or early February on the West Coast and that the virus spread undetected for more than a month.
Scientists from the agency, and research groups they collaborate with, concluded community transmission in the U.S. began with a single unidentified imported case from China.
The CDC team drew on multiple streams of data, including retrospective testing of nearly 11,000 samples collected through CDC-led influenza surveillance networks.
If Covid-19 cases were being mistaken for flu in the early days of the outbreak, one would have expected to have seen some positive tests among those samples. But the first that was positive was collected from a patient in Washington state on Feb. 25 — two days before authorities in California reported finding an infection in a woman who had not traveled outside the country and who was not a known contact of people who returned to the U.S. infected with the virus.