Footnotes

• Contributors Drs SKMG and SC reviewed the research papers and drafted the paper; SGM and BDP critically reviewed the content and significantly edited the manuscript.

• Funding SGM and SKMG are supported by the National Institute for Health Research (NIHR) Biomedical Research Centre,Oxford

• Competing interests None declared.

• Provenance and peer review Commissioned; externally peer reviewed.

• Author note References which include a * have been identified as a key reference for this paper.