1. Department of Environmental and Radiological Health Science, Colorado State University, Fort Collins, CO 80523, USA
2. Africa Health Research Institute, Somkhele, Myeki 3935, South Africa
3. Perinatal HIV Research Unit, Soweto 1862, South Africa
4. School of Global Public Health, New York University, New York, NY 10003, USA
5. Klerksdorp Tshepong Hospital Complex, Matlosana 2574, South Africa
6. Department of Infectious Diseases, Johns Hopkins School of Medicine, Baltimore, MD 21205, USA