We assessed 12,320 women for eligibility and randomly assigned 5029 women to a study group (Fig. 1). The primary exclusion criterion was HIV-1 infection (32%; range, 3 to 53% according to site). A total of 61.8% of the participants were from Durban, South Africa; the majority of participants from this site were unmarried women who were younger, on average, than other participants. The characteristics of the participants were similar among the study groups (Table 1). The mean age was 25.3 years, 21% of the women were married, and 71% used injectable hormonal contraception. The participants reported a mean number of 2.5 episodes of vaginal intercourse during the 7 days before enrollment.