This is a daily updated tracker website for COVIDâ€“19 case counts and transmission rates in California and in the United States.

We estimate transmission rates of COVIDâ€“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 individualâ€™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 publically available daily counts of COVIDâ€“19 cases by county and state, archived by The New York Times from multiple local sources. We estimate the effective reproduction number (*R*) on each day in the San Francisco Bay Area, in the rest of California, and in the 50 US states and the District of Columbia. We use the Wallinga-Teunis technique of real-time estimation of reproduction numbers. For more details, please see our preprint on the medRxiv preprint server.