Fig. 2.

The epidemic curve of daily reported cases of COVID-19 and the corresponding effective reproduction number. The bar in the figure gives the epidemic curve of the original data of reported cases by day during the COVID-19 epidemic in Wuhan in early 2020. The black solid line is the effective reproduction number $R_t$ over time $t$, the size of which is measured by the right y-axis. When $R_t$ is above the threshold value one represented by the horizontal dot-dash line, the epidemic is in an increasing trend; otherwise, it is in decline. The left vertical dashed line with an arrow pointing to $T_1$ (January 23, 2020) is the day when the lockdown started in Wuhan, while the right vertical dashed line with an arrow pointing to $T_2$ (March 24, 2020) is the day when the last confirmed case was reported.