The HSV-2 incidence rate among the 25 women with vaginal tenofovir concentrations of 10,000 ng per milliliter or more was 5.7 cases per 100 person-years , as compared with 15.5 cases per 100 person-years among the 103 women with no detectable vaginal tenofovir ( incidence rate ratio , 0.37 ; 95 % CI , 0.04 to 1.51 ; P = 0.14 ) .