State-by-State estimates of R0 at the start of COVID-19 outbreaks in the USA

We estimated the initial rate of spread (r0) and basic reproduction number (R0) for States in the USA experiencing COVID-19 epidemics by analyzing death data time series using a time-varying autoregressive state-space model. The initial spread varied greatly among States, with the highest r0 = 0.31 [0.23, 0.39] (95% CI) in New York State, corresponding to R0 = 6.4 [4.3, 9.0] (95% CI). The variation in initial R0 was strongly correlated with the peak daily death count among States, showing that the initial R0 anticipates subsequent challenges in controlling epidemics. Furthermore, the variation in initial R0 implies different needs for public health measures. Finally, the States that relaxed public health measures early were not those with the lowest risks of resurgence, highlighting the need for science to guide public health policies.