[1] Kenya Govt Med Res Ctr, Ctr Geog Med Res Coast, Kilifi, Kenya
[2] UCL, Inst Child Hlth, Dev Cognit Neurosci Unit, London WC1E 6BT, England
[3] UCL, Inst Child Hlth, Wolfson Ctr, Neurosci Unit, London WC1E 6BT, England
[4] London Sch Hyg & Trop Med, Clin Res Unit, London WC1, England