[1] Africa Ctr Populat Studies & Reprod Hlth, Hlabisa, South Africa
[2] Univ KwaZulu Natal, Durban, South Africa
[3] MRC, Div HIV Prevent & Vaccine Res, Durban, South Africa
[4] Univ Witwatersrand, Dept Obstet & Gynaecol, Reprod Hlth Res Unit, Durban, South Africa
[5] Univ Cape Town, Womens Hlth Res Unit, ZA-7925 Cape Town, South Africa
[6] Columbia Univ, Sch Publ Hlth, Div Epidemiol, Fogarty AITRP, New York, NY USA
[7] MRC, Biostat Unit, Durban, South Africa
[8] Univ Waikato, Dept Stat, Waikato, New Zealand