Santa Clara County's Chief Executive Dr. Jeff Smith suggests that COVID-19 could have been in the Bay Area and circulating, since late 2019 in December, according to the article in the LA Times.
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That realization is thanks to data collected by the Centers for Disease Control and local health departments. Dr. Smith also said early detection of the virus lagged because of the severe flu season the Bay Area was experiencing and the overlap in symptoms with coronavirus. Then there were the guidelines that only travelers recently in Wuhan, China should be screened. And by that point, it was already spreading within the community.
By late February, when doctors finally tested a hospitalized woman who had been sick for weeks, she became the Bay Area's first case of community spread of the virus. From there almost every positive test pointed to local spread.
The article also points to an overall late start to community surveillance of the coronavirus which led to medical leaders not catching the "community spread" of the illness earlier.
Now researchers are looking at blood banks in Los Angeles and San Francisco among other major cities to see if lingering antibodies can show them what was missed.