HPV is a virus known to cause genital warts and cervical cancer and is the most common sexually transmitted infection in the United States. The HPV vaccine works by preventing the most common types of HPV, and it’s recommended for all teen girls and women younger than 26. It’s most effective when individuals receive all three doses of the vaccine before becoming sexually active.

But a new study is showing that the vaccine not only prevents infections but also promotes what is known as “herd protection”—a decrease in infections among those who are not immunized. This is due to the lower rates of infection among others who might be transmitting the disease.

For the study, researchers looked at two groups of women ages 13 to 26, all of whom had already had sexual contact. One group was seen between 2006 and 2007—before the HPV vaccine was available—and the second group was seen between 2009 and 2010, after vaccines were readily available. About 60 percent of participants in the second group had received the vaccine.

Researchers found that from 2006 to 2010, the prevalence of the four most common strains of HPV decreased by about 60 percent, from about 32 percent to 13 percent. And among individuals who were vaccinated, rates of HPV infection fell from 32 percent to 10 percent. Most important, rates of infection among unvaccinated individuals 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,” said Jessica Kahn, associate professor of pediatrics at Cincinnati Children’s Hospital Medical Center. “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.”

Why did researchers find such a dramatic drop?

“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.

However, although the prevalence of the four main HPV strains fell, the overall rate of HPV infection remained high. Infection by any HPV strain increased from 61 percent to 76 percent during the study’s two time periods, which researchers said highlights the need for vaccines.