1. Section of Infectious Diseases, Wright-Fleming Institute, Faculty of Medicine, Imperial College London, London, UK
2. Department of Genito-urinary Medicine, Princess Grace Hospital, London, UK
3. School of Social and Community Medicine, University of Bristol, UK
4. National Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Evaluation of Interventions in partnership with Public Health England, University of Bristol, UK
5. Bristol Sexual Health Centre, University Hospitals Bristol NHS Trust, Bristol, UK