1. AIDS Virus Research Unit, National Institute for Communicable Diseases, and University of the Witwatersrand, Johannesburg, South Africa.
2. Perinatal HIV Research Unit, Chris Hani Baragwanath Hospital, Soweto, South Africa.
3. National Health Laboratory Services and University of the Witwatersrand, Department of Molecular Medicine and Haematology, Johannesburg, South Africa.
4. Empilweni Clinic, Coronation Women and Children Hospital, Enhancing Childhood HIV Outcomes (ECHO), University of the Witwatersrand, Johannesburg, South Africa.
5. Gertrude H. Sergievsky Centre, College of Physicians and Surgeons and Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, New York.