Every Thursday, SFGATE will report the latest estimates for R0, the base breeding number, for the coronavirus in the nine-county San Francisco Bay Area. Using several different models, the state makes estimates for R0 in each county on its California COVID Assessment Tool (CalCAT) website. For an explanation of what R0 means and how it is calculated, click here.
Earlier in the week, San Francisco health director Dr. Grant Colfax stated that city officials believe that R0, the average number of people infected by a case of coronavirus, is approximately 1.25.
If R0 is significantly greater than 1.0, a county will see an “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 reach a peak of 830 hospitalizations in our city,” he said. “In April, we peaked at 94, and thanks to all the work people did in San Francisco and taking refuge there, we reduced that number.”
It’s unclear how the city calculated its R0 estimate, but state forecasters offer a much lower estimate for San Francisco’s base breeding number. Using models from Johns Hopkins University, Stanford University, and others, the projection of the state “set” for R0 in San Francisco is exactly 1.0. If R0 is 1.0, the spread of the virus is stable.
Here are the R0 state estimates for the nine Bay Area counties, plus the state designation for each.
Mall R0 = 1.08 “The spread of COVID-19 is probably stable.” It was 1.06 on July 9.
Against Coast: R0 = 1.18 “The spread of COVID-19 probably increases.” It was 1.18 on July 9.
Marin R0 = 1.1 “The spread of COVID-19 is probably increasing.” It was 0.89 on July 9.
Nappa R0 = 0.97 “The spread of COVID-19 is probably stable.” It was 1.01 on July 9.
San Francisco: R0 = 1.0 “The spread of COVID-19 is probably stable.” It was 1.06 on July 9.
Saint Matthew: R0 = 1.07 “The spread of COVID-19 is probably stable.” It was 1.02 on July 9.
Santa Clara: R0 = 1.06 “The spread of COVID-19 is probably stable.” It was 0.99 on July 9.
Solano R0 = 1.13 “The spread of COVID-19 probably increases.” It was 1.11 on July 9.
Sonoma: R0 = 1.13 “The spread of COVID-19 probably increases.” It was 0.93 on July 9.
There hasn’t been much movement since the past week with the exception of Marin County, which has seen dramatic changes because of the state using San Quentin State Prison cases in county estimates.
Marin County’s estimated R0 was at 1.42 on June 25, but fell to 0.89 last week. A Marin County spokesman said the county believes the dramatic reduction in R0 would not have happened if the prison numbers had not been part of the total. The county epidemiology team noted that as the number of active cases in San Quintín declined sharply before July 9, that rapid change in active cases over time led to a dramatic reduction in R0. The number of active cases in San Quentin is on the rise again, which will increase the county’s R0 estimates.
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Eric Ting is a digital reporter for SFGATE. Email: [email protected] | Twitter: @_ ericting
