ATLANTA (AP) — Researchers say a vaccine for a sexually spread virus has cut infections in teen girls by half.

This is the first evidence of how well the HPV vaccine works since it came on the market seven years ago.

For girls ages 14 to 19, the study found a 56 percent reduction in the types of HPV virus targeted by the shots. Vaccination campaigns focus on girls ages 11 and 12.

Many men and women are infected with the human papillomavirus during their life. Most don't develop symptoms and clear the infection on their own. But some infections lead to genital warts, cervical cancer and other cancers.

Results of the study by the Centers for Disease Control and Prevention were released Wednesday.