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First Online: 24 February 2017

Compliance with Ethical Standards

:

: Yingxi Chen, Aron Hall and Martyn Kirk declare no conflict of interest.

: This article does not contain any studies with human or animal subjects performed by any of the authors.

: All reported studies/experiments with human or animal subjects performed by the authors have been previously published and complied with all applicable ethical standards (including the Helsinki declaration and its amendments, institutional/national research committee standards, and international/national/institutional guidelines).

: The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.