Multiple published reports now suggest California was exposed to the Covid-19 virus in January and February of 2020. This could have helped the people in the Bay Area develop "herd" immunity. Studies are on-going through Stanford and other institutions to determine if and how many people may have been exposed and are now immune. One interesting way to figure that out is to evaluate blood that has been donated over the last two months for antibodies to the virus. Stay tuned. This is an interesting and evolving story. 
Ref: https://www.rt.com/usa/485586-coronavirus-in-california-longer/