Funding for this research was provided by:
Received: 24 April 2018
Accepted: 15 April 2019
First Online: 2 May 2019
Ethics approval and consent to participate
: The study was approved by the Ethical Review Committee (ERC) of the West China School of Public Health, Sichuan University. Verbal informed consent was obtained from each participant prior to our study, because signing inform consent would cause some undue threat to the subjects’ privacy.
: Not applicable.
: The authors declare that they have no competing interest.
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