Footnotes

• Contributors Xiaohua Zhou and Xin Chen planned the article, discussed content, and wrote and reviewed the article. Lijuan Shen and Shuping Cai discussed content. Hongjian Ji was involved in case collection and helped to write the article, Joe Barber Jr helped to write the English text of a draft of this manuscript.

• Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests None declared.

• Provenance and peer review Not commissioned; internally peer reviewed.