Funding for this research was provided by:
the China Mega-Project for Infectious Diseases (2018ZX10711001,2017ZX10303404-007-002)
National Natural Science Foundation of China (81102171)
the National Key Research and Development Program of China (2016YFC12002)
Natural Science Foundation of Shandong Province (ZR2014HP030)
Received: 14 October 2018
Accepted: 7 May 2019
First Online: 28 May 2019
Ethics approval and consent to participate
: This study was approved by the ethics committee of the National Institute for Viral Disease Control and Prevention, Chinese Center for Disease Control and Prevention. Written informed consent was obtained from the study participants.
: Not applicable.
: The authors declare that they have no competing interests.
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