[1] London Sch Hyg & Trop Med, Dept Populat Hlth, Keppel St, London WC1E 7HT, England
[2] London Sch Hyg & Trop Med, Dept Infect Dis Epidemiol, London, England
[3] Biomed Res & Training Inst, Harare, Zimbabwe
[4] Biomed Res & Training Inst, Dept Clin Res, London, England