1. Department of Genitourinary Medicine, Mortimer Market Centre, Camden PCT, London WC1E 6JB;
2. Department of Genitourinary Medicine/HIV, Brighton and Sussex University Hospitals NHS Trust, Brighton BN1 5BE;
3. Centre for HIV and Sexual Health Research, Department of Primary Care and Population Sciences, Royal Free and University College Medical School, University College London, London WC1E 6JB, UK