[1] Natl Inst Communicable Dis, Ctr HIV & STI, Johannesburg, South Africa
[2] Univ Witwatersrand, Sch Publ Hlth, Johannesburg, South Africa
[3] South African Med Res Council, Biostat Unit, Cape Town, South Africa
[4] South African Med Res Council, Biostat Unit, Pretoria, South Africa
[5] Univ Pretoria, Dept Stat, Pretoria, South Africa
[6] Ctr Dis Control & Prevent, Pretoria, South Africa
[7] Univ Witwatersrand, Sch Pathol, Div Virol, Johannesburg, South Africa