S Lu and H Zhu made a significant contribution to the work reported in the study conception, design, execution, acquisition of data, analysis and interpretation, or in all these areas. S Lu, X Guo, Z Yang, J Niu and X Jing performed data collection and analysis. S Lu and X Guo drafted or wrote the article. S Lu and Z Yang substantially revised or commented on the article. H Zhu reviewed the article prior to submission, during review, acceptance of the final version for publication and on any major revisions introduced at the proofreading stage. H Zhu agreed to be responsible and accountable for the content of the article. All authors agreed on the journal to which the article was submitted. All authors gave final approval of the version to be published and accept responsibility for all aspects of the work. Y Sun substantially revised or commented on the article.