A new study confirms the the effectiveness of a HPV vaccine introduced almost a decade ago in the US; the virus’ prevalence in teenage girls is down by almost two thirds. Even for women in their early 20s, the group with the lowest vaccination rates, infections have been reduced by more than a third.

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The CDC estimates that about 14 million Americans become infected with the human papillomavirus (HPV) each year, making it the most common sexual transmitted infection. The vast majority will naturally fight off the bug, but some strains resist our immune system’s efforts and can cause genital warts, as well as cervical, anal, penile, or mouth and throat cancers. The American Cancer Society estimates that 4,120 women will die of HPV-associated cervical cancer this year.

In 2006, a vaccine that could prep the body to fight was developed and distributed in small amounts throughout the country.

“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.”

Using data from a survey by the Centers for Disease Control and Prevention, the study examined the incidence of the virus in females of different age groups during the pre-vaccine years of 2003 through 2006. Researchers then compared their findings with the prevalence in the same age groups between 2009 and 2012.

By that time, four strains of HPV (all covered by the vaccine) had a 64% decrease in incidence among girls aged 14 to 19, and 34% decline among women aged 20 to 24. For the 25 and older group, no decline in infection rate was seen.

“The vaccine is more effective than we thought,” said Debbie Saslow, a public health expert in HPV vaccination and cervical cancer at the American Cancer Society. As vaccinated teenagers become sexually active, they are not spreading the virus, so “they also protect the people who haven’t been vaccinated,” she said.

The findings are already being used by those advocating for HPV vaccinations in the US. Efforts are being focused on recommending the vaccine for children ages 11 and 12, when their immune response is more robust than that of teenagers and when most states require two other vaccines — one for tetanus, diphtheria and pertussis, and the other for meningococcal disease.

However, there are a few hurdles in getting more people vaccinated.

First of all, while other countries the vaccine comes in two doses (particularly to girls under 15 years of age,) in the US it is administered in three doses. An immunization advisory committee to the C.D.C. has been set up to investigate the efficiency of the lower dose.

The second and more hotly debated issue is that in some countries the vaccine is either mandatory or at least offered at school, with the cost covered by a national health care system. This makes administration more streamlined and comprehensive. Using such a system, Rwanda achieved a 93 percent immunization rate in girls. Australia, where the vaccine is offered free to schoolgirls, accomplished a 92 percent reduction in genital warts in women under 21, a study showed. But in the United States, the vaccine is largely optional and almost always paid for by the parents. Only Virginia, Rhode Island and the District of Columbia have legislation set in place that requires the HPV vaccine.

“Multiple studies have shown the importance of a strong provider recommendation for increasing vaccination coverage,” said Dr. Lauri E. Markowitz, a medical epidemiologist at the National Center for Immunizations and Respiratory Diseases, a division of the C.D.C., lead researcher for the latest study.

Not even that, but the parents aren’t even told they have this option most of the time. A recent study showed that most primary care providers either do not recommend the vaccine to parents and patients or do so halfheartedly. This comes down to the association between HPV and sexual activity: some of the doctors want to talk topics which parents are more willing to engage in, while others just don’t feel comfortable dancing around this personal subject.

“If a parent is concerned, physicians should be prepared to talk about it,” said Dr. Bocchini, a former chairman of an HPV vaccine working group for the committee that advises the C.D.C. on immunizations.

In an effort to shift the focus back to the vaccine’s purpose, dozens of treatment centers endorsed the vaccine as a safe and efficient prevention strategy against cancers associated with the virus.

“The infection is sexually transmitted, but that doesn’t need to be part of the conversation,” said Dr. Joseph A. Bocchini Jr., a pediatric infectious disease specialist at Louisiana State University in Shreveport.

“We don’t really discuss how people become infected with every vaccine-preventable disease,” Dr. Bocchini concluded.

The latest HPV vaccine protects against nine strains of the virus. The study only included females as the recommendation to vaccinate boys became widespread only in 2011; they will be included in subsequent studies.

The full paper, titled “Prevalence of HPV After Introduction of the Vaccination Program in the United States” has been published online in the journal Pediatrics and is available here.