“We be to reproduction number estimation, what key be to lock”

# Introduction

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

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 publically available daily counts of COVID–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.

## Counties/Regions of California