1. Epidemiology and Biostatistics Division, School of Public Health, Faculty of Health Sciences, University of The Witwatersrand, 7 York Road, Parktown, Johannesburg 2193, South Africa
2. Centre for Infectious Disease Epidemiology and Research (CIDER), University of Cape Town, Cape Town 7925, South Africa
3. Liverpool School of Tropical Medicine, Liverpool L35 QA, UK
4. Malawi-Liverpool-Wellcome Trust Clinical Research Programme, Blantyre, Malawi