[1] Univ Calif San Francisco, Dept Obstet Gynecol & Reprod Sci, San Francisco, CA USA
[2] Univ Calif San Francisco, Dept Med, ARI Lab Clin Virol, San Francisco, CA USA
[3] Univ Calif Berkeley, Div Epidemiol, Berkeley, CA 94720 USA
[4] Kenya Govt Med Res Ctr, Ctr Microbiol Res, Nairobi, Kenya