Funding for this research was provided by:
National Natural Science Foundation of China (81373125)
Received: 2 January 2019
Accepted: 10 April 2019
First Online: 25 April 2019
Ethics approval and consent to participate
: This work was approved by the ethical review board of the National Center for AIDS/STD Control and Prevention (Project No. X140617334). All participants have given verbal informed consent.
: Not applicable.
: The authors declare that they have no competing interests.
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