The human papillomavirus is one of the most common sexually transmitted infections, but a new U.S. study shows the HPV vaccine appears to be protecting not just the teens who get the shot but also those who were not immunized.

The study by researchers at the Cincinnati Children's Hospital Medical Center was published online July 9 in the eFirst pages of Pediatrics.

It is believed to be the first study to show a significant decrease in HPV infection in a community setting as well as "herd protection" — a decrease in infection rates among unimmunized individuals that occurs when a critical mass of people in a community is immunized against a contagious disease.

"Infection with the types of HPV targeted by the vaccine decreased in vaccinated young women by 69 per cent," says Dr. Jessica Kahn, a physician in the division of Adolescent Medicine at Cincinnati Children's and lead author of the study.

"Two of these HPV types, HPV-16 and HPV-18, cause about 70 per cent of cervical cancer. Thus, the results are promising in that they suggest that vaccine introduction could substantially reduce rates of cervical cancer in this community in the future."

For the study, Kahn and her colleagues compared two groups of young women between the ages of 13 and 16.

The first group of 368 young women had sexual contact but were not vaccinated and were recruited from clinics in 2006 and 2007.

More than half of the second group of 409 young women recruited in 2009 and 2010 had received at least one dose of the vaccine.

In comparing pre- and post-vaccination HPV prevalence rates, the researchers found that the prevalence of vaccine-type HPV decreased 58 per cent overall, from 31.7 per cent to 13.4 per cent.

The decrease was high among vaccinated participants (69 per cent), but also was substantial for those who were unvaccinated (49 per cent).

The decrease in vaccine-type HPV among vaccinated participants was "especially remarkable," given that participants were sexually experienced, many were exposed to vaccination-type HPV before vaccination, and only one dose of the vaccine was required to be considered vaccinated, said Khan.

Despite the evidence of herd immunity demonstrated in the study, vaccination of all young women between the ages of 11 and 26 is important to maximize the health benefits of vaccination, added Khan.

The first HPV vaccine was licensed for use in the United States in June 2006. The U.S. advisory committee on immunization practices has recommended vaccination of girls and women between the ages of 11 and 26 to reduce rates of HPV infection, which ultimately can lead to cervical cancer. The study was funded by the National Institutes of Health.