Human papilloma virus (HPV) is the most common sexually transmitted infection worldwide. The virus is the primary cause of cervical cancer and can also lead to other genital and head and neck cancers in both women and men.

In fact, about 630,000 new cancer diagnoses each year are attributed to HPV.

HPV rates dropped by almost 76 percent among participants who received the HPV vaccine available in the U.S. It is effective against nine variants of the virus.

HPV vaccines were first introduced 17 years ago. As the results of randomized clinical trials show, they provide protection against the virus. Participants in those trials, however, tended to be healthier and at lower risk for HPV infection than the general population.

A recent study led by researchers at the Albert Einstein College of Medicine found the HPV vaccine was very effective at preventing infections among more than 2,300 adolescent girls and young women between the ages of 13 and 26 whose sexual behavior put them at high risk for HPV.

Seventy-nine percent of study participants had two or more male sexual partners and 51 percent had a history of at least one sexually transmitted infection. Because the young women were sexually active, they may have been exposed to HPV before vaccination. They also may not have received all the doses of the vaccine, Jessica Kahn, lead author on the study, explained.

During the 17-year study period, vaccination rates rose dramatically — from zero to 82 percent. At the same time, HPV rates dropped by almost 76 percent among participants who received the only currently available HPV vaccine in the U.S., which is effective against nine types of HPV.

The effectiveness of the vaccine in this high-risk population surprised Kahn. “The levels of effectiveness we saw in a real-world setting were comparable to levels of effectiveness seen in randomized clinical trials, which was really remarkable,” she told TheDoctor.

The researchers also found strong evidence of herd immunity, which happens when enough members of a community are vaccinated to prevent the spread of infection. Among unvaccinated participants, infections with two types of HPV vaccine decreased by more than 71 percent and infections with four types of HPV decreased by almost 76 percent.

The high rate of herd immunity could be attributed to high vaccination rates and the strategy of vaccinating both boys and girls, said Kahn, a professor of pediatrics and the Dr. Ernest Baden Chair in Head and Neck Pathology at the Albert Einstein College of Medicine. If people don't get vaccinated, however, herd immunity will not occur. “Herd immunity is not guaranteed. A person is really rolling the dice if they decide to rely on herd immunity for protection,” Kahn said.

High vaccination rates and the strategy of vaccinating both boys and girls are believed to have contributed to the degree of herd immunity achieved during the study period.

In order to protect people who cannot get vaccinated for some reason, a certain level of herd immunity needs to be achieved and maintained, Rachel Katzenellenbogen, one of the authors of an editorial on the study, explained.

Although herd immunity definitely improved over time, there was an increase in HPV infections among unvaccinated participants at the last time point in the study. “That shows some relative fragility in trusting your community to protect you,” Katzenellenbogen, chief of adolescent medicine at Indiana University School of Medicine, told TheDoctor.

This study focused on the effectiveness of HPV vaccination and herd immunity in girls. Kahn and her team also have a lot of data in boys that they are still analyzing. “We would like to look at real world data about effectiveness and herd immunity in boys,” she said.

The study and editorial are published in JAMA Pediatrics.