Footnotes

• Contributors YX designed the study and drafted the manuscript. Q-mQ revised the manuscript. Y-xH and S-yZ shaped the discussion and proofread the final version. YX is responsible for the overall content as guarantor.

• 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.