Temporal relationship between outbound traffic from Wuhan and the 2019 coronavirus disease (COVID-19) incidence in China

Zaixing Shi; Ya Fang

Regional variations in the association between outflowing traffic from Wuhan and COVID-19 incidence worth closer examination. Longer time lag between traffic volume and COVID-19 incidence was observed in provinces closer to Wuhan or with more population influx from Wuhan, such as Henan, Jiangxi, Shanghai, Guizhou, Guangdong, and Sichuan. Longer lagged effect does not mean these provinces were less responsive to imported cases. Rather, it means the overall changes in cumulative incidence in these provinces were not only driven by imported cases but also infected cases through local transmissions. For example, the lag was estimated to be 23 days for Henan province, which was severely affected by COVID-19. Although Henan province was among the earliest to implement traffic control, the COVID-19 incidences grew steadily after the traffic control. It is possible that the influx of Wuhan emigrants, mostly people who returned home from work/school, led to significant local transmissions in Henan, which outweighed the imported cases in driving the cumulative incidence to grow for a prolonged period. Given the median incubation period of 4-6 days for COVID-19 [11] , the overall COVID-19 incidence in provinces with lags of 2-3 weeks, or approximately 2-3 times the median incubation period, could be reflective of the second-or third-generation transmissions. Also, provinces with longer lag tend to have higher cumulative incidence, suggesting widespread local transmissions.