[1] Univ Witwatersrand, Fac Hlth Sci, Johannesburg, South Africa
[2] Ctr HIV & STIs, Natl Inst Communicable Dis, Natl Hlth Lab Serv, Johannesburg, South Africa
[3] Univ Witwatersrand, Fac Hlth Sci, Perinatal HIV Res Unit, Johannesburg, South Africa
[4] Soweto Matlosana Ctr HIV AIDS & TB Res, MRC, Johannesburg, South Africa
[5] Univ Witwatersrand, Sch Clin Med, Fac Hlth Sci, Johannesburg, South Africa