Footnotes

• 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.

• Patient consent Not required.

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

• Author note ARD is a staff member of the WHO in Geneva. He alone is responsible for any views expressed in this publication, and they do not necessarily represent the decisions or policies of any third party.