A vaccine introduced a decade ago to combat the sexually transmitted virus that causes cervical cancer has already reduced the virus’s prevalence in teenage girls by almost two-thirds, federal researchers said Monday.

Even for women in their early 20s, a group with lower vaccination rates, the most dangerous strains of human papillomavirus, or HPV, have still been reduced by more than a third.

“We’re seeing the impact of the vaccine as it marches down the line for age groups, and that’s incredibly exciting,” said Dr. Amy B. Middleman, the chief of adolescent medicine at the University of Oklahoma Health Sciences Center, who was not involved in the study. “A minority of females in this country have been immunized, but we’re seeing a public health impact that is quite expansive.”

The news is likely to serve as a welcome energizer in the tumultuous struggle to encourage HPV vaccination in the United States. Despite the vaccine’s proven effectiveness, immunization rates remain low — about 40 percent of girls and 20 percent of boys between the ages of 13 and 17. That is partly because of the implicit association of the vaccine with adolescent sexual activity, rather than with its explicit purpose: cancer prevention. Only Virginia, Rhode Island and the District of Columbia require the HPV vaccine.