[1] Univ N Carolina, Dept Epidemiol, Chapel Hill, NC 27599 USA
[2] Univ Witwatersrand, Dept Obstet & Gyneacol, Reprod Hlth Res Unit, ZA-2050 Wits, South Africa
[3] MRC, Cape Town, South Africa
[4] Dev Res Africa, Durban, South Africa
[5] Univ Witwatersrand, Contact Lab Serv, ZA-2050 Wits, South Africa
[6] Univ Calif San Francisco, AIDS Res Inst, San Francisco, CA 94143 USA