[1] Univ Witwatersrand, Fac Hlth Sci, Sch Clin Med, Johannesburg, South Africa
[2] Univ Nairobi, Dept Hlth Serv, Campus Support Serv, Adm, Nairobi, Kenya
[3] Univ Nairobi, Fac Sci & Technol, Dept Math, Nairobi, Kenya
[4] Kenya Govt Med Res Ctr, Wellcome Trust Res Programme, Nairobi, Kenya
[5] Univ Witwatersrand, Wits Reprod Hlth & HIV Inst, Fac Hlth Sci, Johannesburg, South Africa
[6] Univ Washington, Sch Publ Hlth, Dept Epidemiol, Seattle, WA USA