Footnotes

• Contributors CBD conceived and designed the work. AGD and JCLR drafted the manuscript, AB revised it, and all authors reviewed the current published literature and critically revised it for important intellectual content and approved the final version to be submitted. CBD is the guarantor of the paper, taking responsibility for the integrity of the work as a whole.

• Disclaimer The views expressed are those of the authors and not necessarily those of the National Health Service, National Institute for Health Research, or Department of Health.

• Competing interests None declared.

• Provenance and peer review Commissioned; externally peer reviewed.