Funding for this research was provided by:
Save the Children International (NA)
Received: 6 December 2019
Accepted: 9 July 2020
First Online: 17 July 2020
Ethics approval and consent to participate
: Ethical approval was obtained by Peking University First Hospital Biomedical Research Ethics Committee [(2018) scientific research no. (13)]. All participants gave written consent.
: Not applicable.
: The authors declare that they have no competing interests.