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Article History

Received: 27 February 2018

Accepted: 2 December 2018

First Online: 8 January 2019

Ethics approval and consent to participate

: All procedures conducted in this study involving human participants and materials were approved by the Jiangsu Provincial Center for Disease Control Ethics Committee, reference number: JSCDCLL2013014. Written consent was obtained from their parents or grandparents (when parents worked outside the hometown, grandparents were legal guardians according to the law of China).

: Not applicable.

: The authors declare that they have no competing interests.

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