Footnotes

• Contributors XLG, YFL: substantial contributions to the acquisition, interpretation of data for the work and revising it critically for important intellectual content. CHN: substantial contributions to the analysis and interpretation of data for the work, and revising it critically for important intellectual content. ZA: substantial contributions to conception and design of the work, analysis and revising it critically for important intellectual content. YPMN: substantial contributions to conception and design of the work, interpretation of data for the work and revising it critically for important intellectual content.

• 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.

• Patient consent for publication Not required.

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