Case #'s in SF worsening. A number I follow is @UCSFHospitals case-pos. rate in asymptomatic pts (ie, pts getting cardiac cath or surgery). Was 0.4% last mth; now 2.1%. So person near you at party or store has 1-in-47 chance of having Covid, even if feeling fine. Be careful (1/3)

And SF's overall case-positivity rate (mix of symptomatic & asymptomatic) is about 4%, still relatively low. I can't find published asymptomatic rates for other cities. Based on @UCSF, might assume it's 1/2 of overall test pos. rate. If that assumption is right, then it's...(2/3)

...even more concerning in cities w/ higher overall rates, such as LA (14.3%), Chicago (11.6%) or Houston (11.8%). If asymptomatic rate=half these rates, would mean that asymptomatic person near me in LA has ~1/14 chance of being infected; ~1/17 chance in Chicago or Houston.(3/3)