Over 20 years of studies have shown the female condom is an effective barrier against many common STIs, including HIV. In FDA safety & efficacy studies FC2 was found 95% effective as a barrier contraceptive. With over 20 years of independent safety, effectiveness, and acceptability research other data is available demonstrating an efficacy as high as 98%.*
* G Farr, H Gabelnick, K Sturgen, and L Dorflinger. Contraceptive efficacy and acceptability of the female condom. American Journal of Public Health December 1994: Vol. 84, No. 12, pp. 1960-1964.