Document is current

Article History

Received: 15 October 2017

Accepted: 21 August 2018

First Online: 3 September 2018

Ethics approval and consent to participate

: This project was approved by the Ethics Review Committee of Henan Center for Disease Control and Prevention. Written informed consent was obtained from all participants. Ethics has been respected throughout the whole study period.

: Not applicable.

: The authors declare that they have no competing interests.

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