Footnotes

• Contributors We can confirm that we followed the ICMJE recommendations that state authorship credit requires substantial contributions to the conception or design of the work, or the acquisition, analysis or interpretation of data, and drafting the work or revising it critically for important intellectual content. Final approval of the version submitted/published has been agreed by all authors.

• Funding This study was funded by Public Health England.

• Competing interests None declared.

• Patient consent Not required.

• Ethics approval University of Birmingham.

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