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Every Thursday, SFGATE will report on the most recent estimations for R0 — the base reproduction number — of the coronavirus in the nine-county San Francisco Bay Area. Using several different models, the state makes estimations for R0 in each county on its California COVID Assessment Tool (CalCAT) website. For an explainer on what R0 means and how it is calculated, click here.
Earlier in the week, San Francisco’s director of health, Dr. Grant Colfax, stated that city officials believe that R0, the average number of people who become infected by one coronavirus case, is approximately 1.25.
If R0 is significantly over 1.0, a county will see “exponential spread” of the virus, and Colfax stated that if SF’s R0 remains at 1.25, very bad things could happen.
“If the reproductive rate stays at that 1.25 number … we would hit a peak of 830 hospitalizations in our city,” he said. “In April, we peaked at 94, and thanks to all the work that people did in San Francisco and sheltering in place, we drove that number down.”
It is unclear how the city calculated its R0 estimate, but the state’s forecasters offer a much lower estimate for San Francisco’s base reproduction number. Using models from Johns Hopkins University, Stanford University and others, the state’s “ensemble” projection for R0 in San Francisco is exactly 1.0. If R0 is at 1.0, the spread of the virus is stable.
Here are the state’s R0 estimates for the nine Bay Area counties, in addition to the state’s designation for each.
Alameda: R0 = 1.08 “Spread of COVID-19 is likely stable.” Was 1.06 on July 9.
Contra Costa: R0 = 1.18 “Spread of COVID-19 likely increasing.” Was 1.18 on July 9.
Marin: R0 = 1.1 “Spread of COVID-19 is likely increasing.” Was 0.89 on July 9.
Napa: R0 = 0.97 “Spread of COVID-19 is likely stable.” Was 1.01 on July 9.
San Francisco: R0 = 1.0 “Spread of COVID-19 is likely stable.” Was 1.06 on July 9.
San Mateo: R0 = 1.07 “Spread of COVID-19 is likely stable.” Was 1.02 on July 9.
Santa Clara: R0 = 1.06 “Spread of COVID-19 is likely stable.” Was 0.99 on July 9.
Solano: R0 = 1.13 “Spread of COVID-19 likely increasing.” Was 1.11 on July 9.
Sonoma: R0 = 1.13 “Spread of COVID-19 likely increasing.” Was 0.93 on July 9.
There has not been much movement from last week with the exception of Marin County, which has seen dramatic swings on account of the state using the San Quentin State Prison cases in the county’s estimations.
Marin County’s estimated R0 was at 1.42 on June 25, but went down to 0.89 last week. A Marin County spokesperson said the county believes the dramatic reduction in R0 would not have happened had the prison numbers not been part of the total. The county’s epidemiology team noted that since the number of active cases at San Quentin sharply decreased prior to July 9, that rapid change over time in active cases led to the dramatic reduction in R0. The number of active cases at San Quentin is once again on the rise, which will increase the county’s R0 estimates.
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