1 The Biomedical Research and Training Institute, Harare, Zimbabwe. Electronic address: mm4059@uw.edu.
2 The Biomedical Research and Training Institute, Harare, Zimbabwe; London School of Hygiene and Tropical Medicine, London, UK.
3 MRC International Statistics & Epidemiology Group, Department of Infectious Disease Epidemiology, University of Washington in St. Louis, St. Louis, MO, USA.
4 Baines Occupational Health Services, Harare, Zimbabwe; University of Witwatersrand, Johannesburg, South Africa.