This is a daily updated tracker website providing time-varying estimated reproduction numbers for COVID-19 in California, based on both daily case counts and wastewater readings.

We estimate transmission rates of COVID–19 using reproduction numbers. A time-varying reproduction number, commonly written R, is the average number of cases infected by a given case over the course of that individual’s disease progression. Of particular interest is the question of whether transmission is supercritical, meaning that R > 1, in which case the epidemic can increase in size, or is subcritical, meaning that R < 1, in which case it will fade out. To eliminate a disease locally, it is not necessary to reduce R to zero, only to reduce it below one for a sustained period.

We use raw data from three sources: new cases by county from California Health and Human Services, (“Cases”, below); wastewater readings from the Cal-SuWers network (“Wastewater (C)”) and additional wastewater figures from BioBot Analytics (“Wastewater (B)”).

For more details see the Methods section below.

Recent

R estimates

This is a plot of estimated reproduction numbers in the Bay Area. For the complete timeseries, see the csv data linked below. The latest estimates are as follows: in Alameda County, from NA, 0.96 on September 19, 2023; in Contra Costa County, from NA, 0.99 on September 19, 2023; in Marin County, from NA, 0.97 on September 19, 2023; in Napa County, from NA, 0.99 on September 18, 2023, from NA, 0.97 on September 18, 2023; in San Francisco County, from NA, 1.00 on September 19, 2023; in San Mateo County, from NA, 1.00 on September 19, 2023; in Santa Clara County, from NA, 0.99 on September 19, 2023; in Solano County, from NA, 0.99 on September 18, 2023; in Sonoma County, from NA, 1.01 on September 19, 2023.

This is a plot of estimated reproduction numbers in the Bay Area. For the complete timeseries, see the csv data linked below. The latest estimates are as follows: in Alameda County, from NA, 0.96 on September 19, 2023; in Contra Costa County, from NA, 0.99 on September 19, 2023; in Marin County, from NA, 0.97 on September 19, 2023; in Napa County, from NA, 0.99 on September 18, 2023, from NA, 0.97 on September 18, 2023; in San Francisco County, from NA, 1.00 on September 19, 2023; in San Mateo County, from NA, 1.00 on September 19, 2023; in Santa Clara County, from NA, 0.99 on September 19, 2023; in Solano County, from NA, 0.99 on September 18, 2023; in Sonoma County, from NA, 1.01 on September 19, 2023.