University of the Witwatersrand - Perinatal HIV Research Unit, Johns Hopkins University - Bloomberg School of Public Health, University of the Witwatersrand - Reproductive Health and HIV Research Unit (RHRU), Vanderbilt University - Division of Infectious Diseases, Stellenbosch University, Gates Foundation, University of the Witwatersrand - Reproductive Health and HIV Research Unit (RHRU), affiliation not provided to SSRN, affiliation not provided to SSRN, affiliation not provided to SSRN, affiliation not provided to SSRN, affiliation not provided to SSRN, affiliation not provided to SSRN, University of Cape Town (UCT) - Division of Chemical Pathology, affiliation not provided to SSRN, affiliation not provided to SSRN, affiliation not provided to SSRN, University of the Witwatersrand - Perinatal HIV Research Unit and affiliation not provided to SSRN