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