Funding for this research was provided by:
Bill and Melinda Gates Foundation (OPP1152504)
the China National Key Research and Development Program (2018YFC1315300)
Received: 12 November 2019
Accepted: 11 February 2020
First Online: 19 February 2020
Ethics approval and consent to participate
: Not applicable.
: All of the authors have reviewed and approved the manuscript for publication.
: The authors declare that they have no competing interests.