MONDAY, July 9 (HealthDay News) -- Even among unvaccinated girls and young women, the human papillomavirus vaccine is reducing infections of certain strains of the virus known to cause genital warts and cervical cancer, new research finds.

The study is among the first to show signs that the human papillomavirus (HPV) vaccine not only prevents infections, but it can also promote herd protection -- a decrease in infections among the unimmunized thanks to lower rates of infections among other people in their community who might otherwise be transmitting the disease.

HPV is the most common sexually transmitted infection in the United States, and is thought to be the leading cause of cervical cancer. Certain strains of the virus can cause other health problems as well, including other genital cancers, anal cancer, head and neck cancers, and genital warts, experts say.

The study involved two groups of women aged 13 to 26 who had already had sexual contact and who were seen at two primary-care clinics in Cincinnati, one of which was a teen health center.

One group was seen at the clinic in 2006 or 2007, before the HPV vaccine, which protects against four strains of HPV, was widely available. The second group was seen in 2009 or 2010, after the vaccines were widely available.

About 60 percent of the latter group had received the vaccine.

Participants filled out a questionnaire with demographic information and information about their sexual activity, and were tested for 37 strains of HPV. The vaccine protects against four common strains of HPV that are known to cause warts or cancer. The strains include types 6 and 11, which cause almost all genital warts, and types 16 and 18, which cause about 70 percent of cervical cancers and 90 percent of anal cancers, said study author Dr. Jessica Kahn, associate professor of pediatrics at Cincinnati Children's Hospital Medical Center.

From 2006 to 2010, the prevalence of these four strains decreased by about 60 percent, from about 32 percent to 13 percent.

Among the vaccinated, rates of HPV infection fell from 32 percent to 10 percent -- a 70 percent drop. Perhaps even more dramatic, rates of infection among the unvaccinated fell from 30 percent to 15 percent -- a 50 percent drop.

"This is a first look at how the vaccine is working in a real-world setting," Kahn said. "We were very encouraged to find the rates of HPV fell so dramatically, especially because the girls in the study already had sexual contact, some had more than one sexual partner and some only had one dose to be considered vaccinated."

"That's what you get in the real world," she continued. "You get vaccinated girls who have already had sex, who aren't compliant with the vaccine. It's a messy sample, not the clean sample you get in a clinical trial."

Although encouraging, Kahn noted, the findings may not be applicable nationally. Most participants in the study were low-income black women, many with Medicaid insurance, and all were drawn from two primary-care centers in the same city.

The study, funded by the U.S. National Institutes of Health, is published online July 9 in the journal Pediatrics.

The U.S. Food and Drug administration has approved two HPV vaccines -- Gardasil and Cervarix. The girls in the study were given Gardasil, which protects against four HPV strains; Cervarix prevents two HPV types.

"If girls are getting vaccinated, they are not spreading HPV to male partners, who then don't spread it to other female partners," Kahn explained.

Although the prevalence of the four main strains of HPV fell, the overall rate of HPV infection remained "extremely high," according to the study. Infection by any strain actually increased from 61 percent to 76 percent during the study's two time periods. Researchers said this highlights the need for vaccines, some of which are under development, that also protect from other strains of HPV.

Dr. Walter Orenstein, a member of the American Academy of Pediatrics' committee on infectious diseases and associate director of the Emory Vaccine Center in Atlanta, said the findings strongly suggest the beginnings of herd immunity against HPV.

"It's too early to be definitive, but there is a very strong suggestion that this vaccine not only protects the [people who received the vaccine] but protects the community as well," Orenstein said.

The U.S. Advisory Committee on Immunization Practices recommends that girls and boys get vaccines against HPV at age 11 or 12, before they've had sexual contact, or up until age 26 if they missed that window. The vaccine is given in three doses, each two months apart. Orenstein urged parents to follow the recommend schedule.

"This is a vaccine that can protect against cancer. It's important to vaccinate at the recommended ages, before there is sexual debut," Orenstein said. "Not only can vaccines protect individuals, but the more individuals who are vaccinated, the more likely the overall community is protected as well."

More information

The U.S. Centers for Disease Control and Prevention has more on vaccination schedules.

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