Funding for this research was provided by:
Bill and Melinda Gates Foundation (OPP1149395)
Received: 28 January 2021
Accepted: 15 June 2021
First Online: 29 June 2021
: Ethical approval of the study was issued by the Institutional Review Board of Chinese Center for Disease Control and Prevention. Consent form was obtained before individual data collection and could be provided upon request.
: Not applicable.
: All authors declare that they have no competing interests.