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  • Contributors BC—literature search, article selection, editing of paper summaries, drafting of the manuscript and submission of the manuscript. SA, RA, CM, TJC and JS—article selection, critical appraisal and article summaries. MJR and AJNJ—article selection and editing of the 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.