Footnotes

• Contributors Concept and design: ZD and DL. Acquisition, analysis and interpretation of data: ZD, SZ, HL, XS, CZ, QD, LL, XZ and LJ. Statistical analysis and drafting of the manuscript: ZD. Critical revision of the manuscript for important intellectual content: ZD. Supervision: ZD.

• 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 and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.

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