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Contributors ZF contributed to the study concept and design. ZF and PR acquired, analysed and interpreted the data. ZF performed the statistical analysis and drafted the manuscript. ZF, RP and WW made critical revisions to the manuscript. PR and WW supervised the study.
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; externally peer reviewed.