Received: 23 April 2020
Accepted: 29 May 2020
First Online: 23 June 2020
Ethics approval and consent to participate
: This study was reviewed and approved by the Institutional Review Board of the Chinese Center for Disease Control and Prevention (China CDC). Since no individual-level case information was used in the analysis, no informed consent was required.
: Not applicable.
: The authors declare they have no competing interests.