https://www.mercurynews.com/2020/04/17/coronavirus-2-5-to-4-2-of-santa-clara-county-residents-infected-stanford-estimates/

New Stanford research concludes that an estimated 2.5% to 4.2% of Santa Clara County residents carry antibodies to the COVID-19 virus, revealing that the pathogen is far more widespread than earlier estimated.

This startling finding suggests that between 48,000 and 81,000 people were infected in Santa Clara County by early April, between 50 and 85 times higher than the number of confirmed cases at that date.