Funding for this research was provided by:
Health and Family Planning Commission of Sichuan Province (17PJ440)
Received: 23 April 2019
Accepted: 19 July 2019
First Online: 30 July 2019
Ethics approval and consent to participate
: The study was approved by the Ethical Committee of Sichuan Provincial Center for Diseases Control and Prevention.
: Not applicable.
: The authors declare that they have no competing interests.