Second, our paper adds to the epidemiological studies on the basic reproduction number (R0) of COVID-19, i.e., the average number of cases directly generated by one case in a population where all individuals are susceptible to infection. Given the short time period since the beginning of the COVID-19 outbreak, research is urgently needed to assess the dynamics of transmissions and the implications for how the COVID-19 outbreak will evolve (Wu et al. 2020b, 2020c). Liu et al. (2020) identify 12 studies that estimated the basic reproductive number in the wide range of 1.4 to 6.5 (with a mean of 3.28 and a median of 2.79) for Wuhan, Hubei, China, or overseas during January 1 through January 28, 2020.4 Our R0 estimate relies on spatially disaggregated data during an extended period (until February 29, 2020) to mitigate potential biases, and the instrumental variable approach we use isolates the causal effect of virus transmissions and imposes fewer restrictions on the relationship between the unobserved determinants of new cases and the number of cases in the past. Simultaneously considering a more comprehensive set of factors in the model that may influence virus spread, we find that one case generates 2.992 more cases within 2 weeks (1.876 if cities in Hubei province are excluded) in the sub-sample from January 19 to February 1. In the sub-sample from February 2 to February 29, the transmission rates fall to 1.243 (0.614 excluding Hubei province). Our estimate of R0 for the period in late January 2020 that overlaps with existing studies falls well within the range of the estimated R0 in the emerging COVID-19 literature (Liu et al. 2020).