⏩ S.F. surge:
Experts cite population density as possible reason for city’s higher infection rate.
San Francisco’s coronavirus case rate hit a pandemic high over the past week, days before public schools open.
For nearly 16 months, the city consistently had one of the lowest infection rates in the nation. But that started to change in mid-July when San Francisco’s case rate surpassed that of the Bay Area as a whole.
Now the city’s COVID-19 case rate has risen above that of California.
With 78% of the eligible population fully vaccinated, the San Francisco Department of Public Health cited density and frequent testing in neighborhoods with higher-than-average infection rates as two main factors behind the sudden surge.
“While increases in cases were expected when we reopened our economy in alignment with the state on June 15, the delta variant has definitely brought new challenges,” the department said in a statement.
As of Wednesday, San Francisco’s rate was 28 per 100,000 residents — higher than the rates of all other Bay Area counties except Contra Costa,
which recorded 29 per 100,000. It’s significantly higher than the average case rate of 19 per 100,000 for the region as a whole, according to data analyzed by The Chronicle.
The case rate rose by a factor of about 7 between July 5 and August 5, according to city data.
Experts agreed that the concentration of people — San Francisco is more densely populated than every other U.S. city except New York — could be a factor.
“We know density does matter,” said Kirsten Bibbins-Domingo, chair of the department of epidemiology and biostatistics at the University of California San Francisco. “It matters where we live, how we interact. Our margins to accommodate the issues of greater density are even less when you have a highly contagious delta variant that needs a shorter period of time to be transmitted from one person to another.”
Peter Chin-Hong, an infectious disease expert with UCSF, said that the virus thrives on the “proximity of noses and mouths together.” So “when you have that highly dense population all clustered together in a peninsula it’s no surprise you have that happen.”
San Francisco’s seven-day rolling coronavirus test positivity rate in San Francisco hit 5.8% last Wednesday, the highest it has been since April 2020. It surpassed the 5.4% peak of the second surge reached in January, and is nearly double the 3.7% peak recorded last summer.
It is possible the surge is starting to ease, but it’s hard to predict.
“There’s some indication that things are starting to level off a little bit but I think it’s too early to really determine whether that’s going to be maintained or not,” Health Director Grant Colfax said Thursday at a news conference announcing that the city would become the first in the nation to mandate full vaccination for indoor activities such as going to bars, restaurants and gyms, though not shopping.
The good news is that even though San Francisco has the highest case rate in the Bay Area, it has one of the lowest hospitalization rates among U.S. counties with comparable transmission rates. That means the vaccines are working and could be a more relevant metric when talking about the status of the virus in San Francisco.
“There’s where we have everyone beat because many of our cases are not turning into hospitalizations,” said ChinHong. “San Francisco has done a lot of outreach into populations that might have done poorly in California. They’ve reached out into many of these neighborhoods, done vaccinations, educated people, ramped up testing — these are all reasons why I think our ratio is really impressive.”