The HSV-2 incidence rate was 10.2 cases per 100 person-years ( 95 % confidence interval [ CI ] , 6.8 to 14.7 ) among 202 women assigned to tenofovir gel , as compared with 21.0 cases per 100 person-years ( 95 % CI , 16.0 to 27.2 ) among 222 women assigned to placebo gel ( incidence rate ratio , 0.49 ; 95 % CI , 0.30 to 0.77 ; P = 0.003 ) .