As the coronavirus disease 2019 (COVID-19) pandemic is progressing rapidly, many countries implemented control measures to contain the COVID-19 outbreak. In this study, we estimated a time-varying reproduction number, Rt, to assess the effectiveness of control measures employed in 10 selected counties. The time-varying reproduction number, Rt, is the average number of secondary cases of disease caused by an infected individual over his or her infectious period. A value of Rt greater than the threshold of 1 indicates that the infection could be able to spread in the population at time t, whereas Rt< 1 indicates that the epidemic size is shrinking at time t. The goal of the control measures is, therefore, to reduce Rt< 1. Close monitoring of Rt over time can provide a real-time assessment on the effectiveness of control measures and guide adjustments in intervention strategies.

We employed a method proposed by Cori et al. 1 to estimate Rt in 10 selected countries, namely Belgium, China, France, Germany, Iran, South Korea, Spain, Thailand, USA and UK, using the EpiEstim Microsoft Excel spreadsheet (available at http://tools. epidemiology. net/EpiEstim. xls). This Rt estimation method only requires the number of daily new cases and the distribution of the serial interval, which is the time between the onset of