Funding for this research was provided by:
National Major Science and Technology Projects of China (2012ZX10002-001-005, 2012ZX10002-001-005-001)
Received: 2 January 2020
Accepted: 3 June 2020
First Online: 12 June 2020
Ethics approval and consent to participate
: The study protocol conformed to the ethical guidelines of the 1975 Declaration of Helsinki and was approved by the Ethics Committee of the Chinese Center for Disease Control and Prevention. The purpose of the study and the right to information were explained to the participants by research staff. Written informed consent was obtained from each participant before the interview and venous blood collection.
: Not applicable.
: The authors declare that they have no competing interests.