[1] Univ Calif San Francisco, Dept Obstet Gynecol & Reproduct Sci, San Francisco, CA 94143 USA
[2] Univ Calif San Francisco, Ctr AIDS Prevent Studies, San Francisco, CA 94143 USA
[3] Univ Calif San Francisco, Ctr Epidemiol & Biostat, San Francisco, CA 94143 USA
[4] Kenya Govt Med Res Ctr, Ctr Microbiol Res, Kisumu, Kenya
[5] Univ Illinois, Div Epidemiol & Biostat, Chicago, IL 60680 USA
[6] Univ Nairobi, Dept Stat, Nairobi, Kenya