Received: 24 April 2018
Accepted: 24 September 2018
First Online: 3 October 2018
Ethics approval and consent to participate
: This study was approved by the Institutional Review Board and Human Research Ethics Committee of Beijing Center for Disease Prevention and Control. The agreement and verbal informed consent was obtained. The children and their families about the study prospectively were informed and asked for their consent.
: Not applicable
: The authors declare that they have no competing interests.
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