1. Department of Clinical Sciences, Institute of Tropical Medicine , Antwerp, 2000 , Belgium
2. Facultad de Medicina (Faculty of Health Science), Universidad de Las Américas , Quito, 170125 , Ecuador
3. Division of Infectious Diseases, Helen Joseph Hospital , Johannesburg, 2092 , South Africa
4. Perinatal HIV Research Unit (PHRU) at the University of the Witwatersrand , Johannesburg, 1864 , South Africa
5. Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine London , London, WC1E 7HT, UK