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Contributors JPE-A and AJR-M conceived the idea and the study design. CAM-C and JPE-A collected the data. LEA-A, MAB-S, DKB-A and AJR-M analysed the data. AJR-M wrote the first draft. All the authors read and approved the subsequent versions and the final submitted 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.
Map disclaimer The depiction of boundaries on this map does not imply the expression of any opinion whatsoever on the part of BMJ (or any member of its group) concerning the legal status of any country, territory, jurisdiction or area or of its authorities. This map is provided without any warranty of any kind, either express or implied.
Competing interests None declared.
Provenance and peer review Not commissioned; internally peer reviewed.