Here is the latest data on virus deaths on a Bay Area county-wide basis (presented in a form that highlights the actual risk of dying thus far in the pandemic) provided by the state of California and the counties themselves.
Alameda, one death per 72,478 people.
Contra Costa, one death per 104,545 people.
Marin, one death per 26,000 people.
San Francisco, one death per 58,866 people.
San Mateo, one death per 36,666 people.
Santa Clara, one death per 32,300 people.
Those numbers are calculated by dividing a county's total population by the number of current deaths due to the virus. As you can see, at least for now, Contra Costa County is by far the safest (or, if you prefer, the least affected) county in the Bay Area.