1. a Department of Epidemiology & Population Health , London School of Hygiene & Tropical Medicine , London , UK
2. b Centre for Sexual Health & HIV Research , Royal Free & University College Medical School , London , UK
3. c Department of Community Medicine , University of Zimbabwe , Harare , Zimbabwe
4. d Department of Infectious & Tropical Diseases , London School of Hygiene & Tropical Medicine , London , UK
5. e Biomedical Research and Training Institute , Harare , Zimbabwe