The following direction has been issued by the Archdiocese of San Francisco:

There is to be no cancellation of regularly-scheduled Sunday and weekday Masses (the latter very rarely if ever draw more than 250 people). For us as Catholics, Sunday worship is essential. It should be noted, too, that the Health Officer of the San Francisco Department of Public Health, Tomás J. Aragón, in his Order No. C19-05 issued on Wednesday, explicitly mentions that places of worship are not included among “Mass Gatherings”.
 
Parishioners who are ill should stay home. This is a situation in which one is not at fault for not attending Sunday Mass. On the contrary, it is the responsible thing to do, out of concern for both one’s own health and that of one’s fellow parishioners. This also applies to those whose condition of health makes them especially vulnerable to infection by the COVID-19 virus. Parishioners confined to their homes on Sunday are encouraged to follow the televised Mass, and to make an act of Spiritual Communion. In addition, there are other ways for homebound parishioners to enter into the spirit of observing the Lord’s Day, such as meditating on the readings for that Sunday, praying the rosary, and (for those who have the availability) praying the Liturgy of the Hours. (Mass is broadcast each Sunday morning at 6:00AM on KOFY 13 and KTSF 26, at 6:30AM on KTXL 40 Sacramento, as well as on EWTN.)