Funding for this research was provided by:
National Major Science and Technology Projects of China (2017ZX10304402-001-015)
Chinese Center for Disease Control and Prevention (59911905)
Received: 1 February 2021
Accepted: 16 April 2021
First Online: 29 April 2021
: This study was approved by the ethics committee of the China Center for Disease Control and Prevention and conducted in accordance with the approved guidelines.
: Not applicable.
: The authors declare that they have no competing interests.