Footnotes

• Handling editor Anna Maria Geretti

• Contributors BB had the original idea, prepared the data and drafted the first version. FR prepared the data and participated in writing and revising the manuscript. JTH reviewed the manuscript drafts and provided feedback. BB and FR prepared the final manuscript. All authors had access to data. All authors have approved the final version.

• 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.