Footnotes

• Contributors FSdL, JT, NI and KS conceived the study, interpreted the data and wrote the manuscript. RW, JD, SK, FEA, SW and SR collected the data and reviewed the literature and the manuscript for intellectual context. PW critically reviewed the manuscript for intellectual content.

• 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.