Funding for this research was provided by:
The National Twelfth Five-year Mega-Scientific Projects of infectious diseases of China (2013ZX10003004-001)
Received: 13 June 2019
Accepted: 9 December 2019
First Online: 31 January 2020
Ethics approval and consent to participate
: The study protocol was approved by the Research Ethics Review Committee of Chinese Center for Disease Control and Prevention (Approval number: 201322). The signed informed consent was obtained from all participants before investigation.
: Not applicable.
: The authors declare that they have no competing interests.