The state of California has launched a new data page full of estimations for the level of coronavirus transmission in every county, as well as projections for future deaths and hospitalizations.

Using estimates from models from UCLA, Imperial College London and more, the state calculates the current base reproduction number — often referred to as R “naught,” R0, or R-effective — in each county. R0 represents the expected number of additional cases generated by one known case in a population where all individuals are susceptible to infection. If R0 is significantly over 1, a county will see “exponential spread” of the virus.


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 “Spread of COVID-19 is likely stable.”



Contra Costa: R0 = 1.29 “Spread of COVID-19 likely increasing.”

Marin: R0 = 1.42 “Spread of COVID-19 is likely increasing rapidly.”

Napa: R0 = 1.18 “Spread of COVID-19 likely increasing.”

San Francisco: R0 = 1.06 “Spread of COVID-19 is likely stable.”

San Mateo: R0 = 1.13 “Spread of COVID-19 likely increasing.”

Santa Clara: R0 = 1 “Spread of COVID-19 is likely stable.”


Solano: R0 = 1.14 “Spread of COVID-19 likely increasing.”

Sonoma: R0 = 1.16 “Spread of COVID-19 likely increasing.”

Of the six counties where the state projects increased spread, Marin, Napa, Sonoma and Solano counties are expected to see a dramatic rise in hospitalizations and deaths according to the state’s model aggregations — but there’s reason to be cautious about those projections. The state uses projections from Johns Hopkins, UCLA and COVIDActNow to create an “ensemble projection” for each county’s outlook, but there are no UCLA projections for the four North Bay counties, leaving just the Johns Hopkins and COVIDActNow projections.


Analysis from SFGATE earlier this week showed Marin is the lone Bay Area county that has seen an increase in both hospitalizations and the percentage of positive tests over the past two weeks. A large outbreak was reported at San Quentin State Prison, but county health officer Dr. Matt Willis said the prison cases are not counted in the county case totals. Willis said outbreaks have been reported at about a dozen Marin County businesses. A county spokesperson told SFGATE that while the San Quentin cases are not part of the county data — meaning they do not explain the recent hospitalization and test positivity increases — the state’s models did use the San Quentin figures in their projections.



Marin County's projected COVID-19 hospitalizations. Photo: California COVID Assessment Tool


Photo: California COVID Assessment Tool


Marin County’s projected COVID-19 hospitalizations.


According the state’s projection, Marin County could see 115 COVID-19 hospitalizations on July 13 — a figure that would represent an 850% increase from the current hospitalization total of 12. Before panicking, Marin County residents should take notice of the fact that the ensemble projection gives significant weight to the Johns Hopkins model, which has consistently been projecting far more hospitalizations than actual hospitalizations. For example, the chart above shows that Johns Hopkins projected Marin County would have 80 hospitalizations on June 23 as opposed to 12. The John Hopkins model is not used to calculate R0, and is only used to project hospitalizations and deaths. It is also unclear why the state ensemble’s hospitalization projections have a rapid drop-off between July 13 and July 17. The projections for Solano, Sonoma and Napa counties look similar.


Contra Costa County has the second-highest estimated R0 in the Bay Area, but the ensemble projection does not predict a dramatic rise in hospitalizations due to the fact that the UCLA model counteracts the Johns Hopkins model. The state ensemble projects hospitalizations in Contra Costa County will remain flat through July.



Contra Costa County's projected COVID-19 hospitalizations. Photo: California COVID Assessment Tool


Photo: California COVID Assessment Tool


Contra Costa County’s projected COVID-19 hospitalizations.


In addition to R0 estimates and hospitalization projections, the new state website also lets users model different scenarios based on the easing of restrictions over time. You can play with it yourself here.

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Eric Ting is an SFGATE digital reporter. Email: eric.ting@sfgate.com | Twitter:@_ericting





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