Funding for this research was provided by:
the Capital Health Research and Development of Special (2014-1-1011)
Text and Data Mining valid from 2018-03-20
Received: 26 September 2017
Accepted: 28 February 2018
First Online: 20 March 2018
Ethics approval and consent to participate
: This study was approved by the Ethics Committee and Beijing Center for Disease Prevention and Control. Written informed consent was obtained from the participants or the guardians if participants were under 16 years of age.
: Not applicable.
: The authors declare that there are no conflicts of interest.
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