aMRC Tropical Epidemiology Group, London School of Hygiene & Tropical Medicine, London, UKbMwanza Intervention Trials Unit, Mwanza, Tanzania; Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, UKcSchool of Public Health, UC Berkeley, USAdMRC Clinical Trials Unit, London, UKeDepartments of Global Health and Medicine, University of Washington, Seattle, USA; Clinical Research Division, Fred Hutchinson Cancer Research Center, Seattle, USA.