1. Centre for Sexual and Reproductive Health Research, Department of Public Health and Policy, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT UK;
2. Department of Public Health and Policy, London School of Hygiene and Tropical Medicine, United Kingdom
3. Department of Primary Care and Population Sciences, University College London, UK
4. Department of Public Health and Policy, London School of Hygiene and Tropical Medicine, UK
5. National Centre for Social Research, London, UK
6. University College London and Department of HIV and Sexually Transmitted Infections, Health Protection Agency, Communicable Disease Surveillance Centre, London, UK