Footnotes

• Contributors GT designed the study. AM, AC and GT did the literature search, with arbitration by AC. AM and GT analysed the data and did the figures. All authors interpreted the data. GT and CDB wrote the manuscript draft. All authors reviewed and approved the final version 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; externally peer reviewed.

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