[1] Univ Birmingham, Inst Appl Hlth Res, Birmingham, W Midlands, England
[2] Univ Bristol, Sch Social & Community Med, Bristol, Avon, England
[3] Publ Hlth England, London, England
[4] MRC, Biostat Unit, Cambridge CB2 2BW, England
[5] Univ Hosp Bristol NHS Fdn Trust, Bristol Sexual Hlth Ctr, Bristol, Avon, England