The earliest date of symptom onset among the 25 patients with a clear date of onset, was January 10th (for 3 patients, the date of onset was unclear). Since then, the number of confirmed index cases imported from abroad continued to increase, and symptoms were first confirmed in patients with secondary infection on January 26th. The incubation period was estimated to be 4.1 days based on the date of symptom onset and first exposure (among 9 patients, excluding 1 patient with an unclear date of onset) shown in Figure 1 and Table 2.