[1] Univ Witwatersrand, Perinatal HIV Res Unit, Johannesburg, South Africa
[2] Univ Cape Town, Sch Publ Hlth & Family Med, Fac Hlth Sci, Cape Town, South Africa
[3] Univ Cape Town, Sch Publ Hlth & Family Med, Hlth Econ Unit, Fac Hlth Sci, Cape Town, South Africa
[4] Univ Witwatersrand, Sch Publ Hlth, Johannesburg, South Africa
[5] Univ Cambridge, MRC Epidemiol Unit, Cambridge, England