Medical Research Council Clinical Trials Unit, University College London, London, UK. Kenya Medical Research Institute, Wellcome Trust Research Programme, Kilifi, Kenya. Malawi Blood Transfusion Services, Blantyre, Malawi. Uganda Blood Transfusion Services, Kampala, Uganda. Department of Paediatrics and Child Health, School of Medicine, Makerere University, Uganda. Busitema University Faculty of Health Sciences, Mbale Regional Referral Hospital, Mbale, Uganda; Mbale Clinical Research Institute, Mbale, Uganda. College of Medicine, Malawi-Liverpool-Wellcome Research Programme, Blantyre, Malawi. Soroti Regional Referral Hospital, Soroti, Uganda. Kenya Medical Research Institute, Wellcome Trust Research Programme, Kilifi, Kenya; Department of Infectious Disease, Institute of Global Health and Innovation, Imperial College London, London, UK. Liverpool School of Tropical Medicine, Liverpool, UK. Haematology and Transfusion Service, Centre Hospitalier et Universitaire, Yaounde, Cameroon. Kenya Medical Research Institute, Wellcome Trust Research Programme, Kilifi, Kenya; Department of Infectious Disease, Institute of Global Health and Innovation, Imperial College London, London, UK. Electronic address: k.maitland@imperial.ac.uk.