1. Gertrude H. Sergievsky Center, College of Physicians and Surgeons, Columbia University, New York, New York.
2. Perinatal HIV Research Unit, Chris Hani Baragwanath Hospital, University of the Witwatersrand, Johannesburg, Johannesburg, South Africa.
3. Empilweni Services and Research Unit, Rahima Moosa Mother and Child Hospital, Department of Paediatrics and Child Health, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, Johannesburg, South Africa.
4. Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, New York.
5. Department of Pediatrics, College of Physicians and Surgeons, Columbia University, New York, New York.
6. ICAP at Columbia University, Mailman School of Public Health, Columbia University, New York, New York.