DISCLOSURE
FUNDING: Chao-feng Chen is supported by Hangzhou Science and
Technology Sanitation Project (2016A02).
CONFLICTS OF INTEREST: The authors have no conflict of interest
to declare.
ETHICS: There were not any ethics problems in our paper.
ACKNOWLEDG EMENTS: Miao-fu Li and Jing Wu: contributed to write
the manuscript; Chao-feng Chen contributed to design of this work,
statistical analysis and write article; Mei-jun Liu: contributed to
evaluate quality and retrieved the required data, Chao-feng Chen:
contributed to review the literature, performed the selection of the
studies and helped gather references for the manuscript. Dr Yi-zhou Xu:
contributed to design of this work, revised and approved the final
version of the manuscript.