Funding for this research was provided by:
Bill and Melinda Gates Foundation (OPP1137180)
Received: 30 November 2020
Accepted: 9 February 2021
First Online: 10 March 2021
Ethics approval and consent to participate
: The study was approved by the Institutional Review Board of the Chinese Center for Disease Control and Prevention .
: Not applicable.
: The authors declare that they have no competing interests.