Footnotes

• Handling editor Anna Maria Geretti

• Contributors FF and DR produced the study concept. FF collected and analysed the data. DR wrote the first draft of the manuscript and created the figure. FF, MTSR, EB, LA, SM, EH and DR all contributed to the final manuscript. FF is acting as the guarantor.

• 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.

• Map disclaimer The inclusion of any map (including the depiction of any boundaries therein), or of any geographical or locational reference, does not imply the expression of any opinion whatsoever on the part of BMJ concerning the legal status of any country, territory, jurisdiction or area or of its authorities. Any such expression remains solely that of the relevant source and is not endorsed by BMJ. Maps are provided without any warranty of any kind, either express or implied.

• Competing interests None declared.

• Provenance and peer review Not commissioned; internally peer reviewed.