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The effect of human mobility and control measures on the COVID-19 epidemic in China

Our take —

Mobility out of Wuhan led to strong transmission chains in other parts of China. The strict ban on travel in or out of Wuhan reduced transmission in the early stage of the outbreak, when importation was the dominant source of new cases. Findings confirm existing knowledge and will not be helpful in settings where the epidemic is well beyond the early stages, or where such severe travel restrictions are impractical.

Study design

Ecological; Modeling/Simulation

Study population and setting

The Chinese government issued a strict ban on travel into or out of Wuhan on January 23, 2020 (the “cordon sanitaire”). Other travel restrictions and suppression measures, not enumerated here, were implemented in close succession. To assess the effect of the Wuhan travel ban on human movements and COVID-19 spread, the authors built a line list dataset of COVID-19 cases in China from December 1, 2019 to February 10, 2020, including information on travel history and demographic characteristics, by extracting individual-level data from official Chinese reports (provincial, municipal, national health governments). Proprietary indices of aggregated human mobility out of Wuhan, and the proportions going to different provinces, were extracted from the Baidu Qianxi web platform.

Summary of Main Findings

The mean incubation period was estimated to be 5.1 days (SD=3.0). The early case count (before February 10, 2020) was tightly associated with the volume of human movement out of Wuhan (R2=0.89). Cases exported from Wuhan before implementation of the cordon sanitaire measures appear to have contributed to initiating local chains of transmission in other provinces. Correlation between daily case counts and human mobility from Wuhan decreased once the cordon sanitaire was put in place, suggesting the increased relative importance of local transmission, and thus of local mitigation strategies.

Study Strengths

To validate the individual-level extracted data, authors conducted sensitivity analyses using reported case counts from the World Health Organization. Estimates of doubling time and incubation period are similar to those found in other studies.

Limitations

The multiple nearly simultaneous interventions make it difficult to single out the effects of the cordon sanitaire. Case definitions changed during the course of the epidemic in China. The human movement data collected from the Baidu Qianxi web platform do not represent the exact number of individual travelers; instead, they represent an index calculated through Baidu’s proprietary methods. Symptom onset date was only available for 667 cases; onset dates for the rest (n=31,436) were estimated using a linear regression model. China’s ability to implement draconian restriction measures may not be feasible in other settings. Findings may only be helpful for countries at early stages of the epidemic.

Value added

The conclusions are not novel, but the methods are strong and convincing.

This review was posted on: 27 April 2020