The HPV vaccine doesn't just help prevent infection among women who get the shot. It may also protect those who don't.

A new study suggests that "herd immunity" -- the idea that once enough people in a population get vaccinated, the protective benefits extend to those who can't or won't -- may help decrease rates of human papillomavirus, the most common sexually transmitted infection in the U.S. The virus affects more than half of sexually active people at some point in their lives.

"Rates of HPV fell by about 50 percent, even in the unvaccinated," said researcher Dr. Jessica Kahn, of the Cincinnati Children's Hospital Medical Center. "That surprised us. We didn't expect that significant a change."

Kahn's research, published in the journal Pediatrics, involved recruiting two groups of approximately 400 women from ages 13 to 26 from two community clinics in Ohio, all of whom reported genital-to-oral or genital-to-genital contact with a male or female partner. Some 60 percent of the women received at least one dose of the HPV vaccine, which protects against many types of the virus, but not all.

In four years, the rates of HPV types not prevented by the vaccine increased. But the prevalence of HPV types that are prevented by the vaccine fell by about 70 percent in women who got the shots, and by 50 percent in those who didn't.

"It was encouraging that the rates declined so much in vaccinated women, because our study was done in a real-world setting, and not in clinical trial," Kahn said. Because usage was not as tightly controlled as it would be in a clinical test, some of the women received only one dose or shot, even though the recommended treatment is three doses over six months.

Kahn said she was also heartened by the suggestion that herd immunity had kicked in.

"That means that young women who are vaccinated are then not transmitting [HPV] to their partners, who are then in turn not passing it on to other partners," Kahn explained. "And the young women who are lesbians are not transmitting it directly."

But experts cautioned against applying the findings too broadly.

"This population of women may be socially intertwined," said Dr. Sharmila Makhija, chair of obstetrics, gynecology and women's health at the University of Louisville, who did not contribute to the study. In other words, because researchers focused on a relatively small population in which many women might share partners, the effect on the spread of the virus may have been much bigger than it would be in a larger group, where people don't mix as much.

In addition, the 60 percent vaccination rate among young women in the study was higher than national estimates. For example, a recent Centers for Disease Control and Prevention survey that just under half of people aged 13 to 17 got one dose of the HPV vaccine. (The two vaccines approved by the Food and Drug Administration -- Cervarix and Gardasil -- are now recommended for girls and boys at age 11 to 12, and up to age 26 in women who are not immunized earlier.)

"For contagious diseases that spread from one person to another, it's possible to immunize a critical percentage of the population, so you effectively decrease the spread," said Dr. Paul Offit, chief of infectious diseases at The Children's Hospital of Philadelphia. "You can have enough people around you immunized, so that you're effectively immunized."

Knowing exactly what that threshold is, however, is tricky. If there is a magic number of people vaccinated that creates HPV herd immunity, that number is far from clear.

In the meantime, experts cautioned that the new findings should not keep women from getting vaccinated, thinking they'll be protected by others who do. HPV infection can be serious -- virtually all cervical cancers are caused by it, according to the National Cancer Institute, as well as almost half of vaginal and vulvar cancers.

"I think it's great knowledge to have [and] that we're seeing that this might be a trend, potentially," said Makhija. "But we have to be cautious that it's in a very select group of women who were socially connected, and that we might not see this same effect on the national scale. I still advocate that we need to vaccinate for this group, for sure."