[1] Kenya Govt Med Res Ctr, Wellcome Trust Res Programme, Malaria Publ Hlth & Epidemiol Grp, Ctr Geog Med Res Coast,Wellcome Trust Res Labs, Nairobi, Kenya
[2] London Sch Hyg & Trop Med, Dept Infect & Trop Dis, London WC1, England
[3] Univ Oxford, Ctr Trop Med, Oxford, England
[4] World Bank, Human Dev Div, Washington, DC 20433 USA