[1] Univ Oslo, Fac Med, Inst Basic Med Sci, N-0372 Oslo, Norway
[2] Univ Cape Town, Med Res Council, Hlth Syst Res Unit, ZA-7925 Cape Town, South Africa
[3] Univ Cape Town, Adolescent Hlth Res Unit, ZA-7925 Cape Town, South Africa
[4] Univ Ghent, Int Ctr Reprod Hlth, B-9000 Ghent, Belgium
[5] Muhimbili Univ Hlth & Allied Sci, Dept Epidemiol & Biostat, Dar Es Salaam, Tanzania
[6] London Sch Hyg & Trop Med, Dept Infect Dis Epidemiol, MRC Trop Epidemiol Grp, London WC1, England