Researchers turned to an insurance database of 12-to-18-year-old girls enrolled in 50 medical plans across the country. About 21,000 vaccinated girls were matched with more than 186,000 non-vaccinated girls, and researchers compared their rates of chlamydia, gonorrhea, herpes, HIV/AIDS and syphilis.
The vaccinated girls had higher STD rates, even a year before getting vaccinated. So researchers looked at the rate of change between the two groups to see whether the vaccines were associated with any change in sexual behavior, said Anupam Jena, assistant professor at Harvard Medical School.
"If anything, we would have expected to find the rates would go up faster in the vaccinated population, if you think the girls who are getting vaccinated have a baseline higher propensity to have sex," Jena said. "But we didn't find that at all. Nothing to me suggests that sexual activity is increasing."
There were some limitations to the study; for instance, researchers didn't have data on the extent of actual sexual activity, such as number of partners or whether condoms were used. But such information gathered via survey questions isn't as reliable as medical data on infections, which for this study served as a way to quantify risky sexual behavior, Jena said.
HPV, spread via sexual activity, can cause cervical cancer in women, as well as other cancers and warts in men. The Centers for Disease Control and Prevention now recommends that girls and boys be vaccinated between 11 and 12 years old, before they become sexually active. There are two vaccines, and they come in three doses.
But despite evidence showing that the vaccine can cut infection rates by half, many people aren't getting their kids vaccinated; 57 percent of girls 13 to 17 years old have received at least one shot, while only about one-third have received all three, according to an ongoing CDC survey.
Some of the resistance to the HPV vaccine involves the relative newness of the shots -- first introduced to the market in 2006 -- and questions about their safety. But the low vaccination rates are also tied to a cultural fear of vaccines, underscored by the current measles outbreak, Jena said.
More than 100 people across the country now have measles in the outbreak tied to unvaccinated children visiting Disneyland. The virus's resurgence has come as many parents have chosen not to vaccinate their children, health officials have said.
The HPV vaccine was contentious even before it was introduced, with social conservatives worried that making the vaccine mandatory would serve as an implicit endorsement for teens to engage in sexual activity, or undermine abstinence messages.
States began considering legislation making the vaccine mandatory. Presidential primary politics even entered the fray during the 2012 campaign season when then-Texas Gov. Rick Perry (R) was criticized for his order requiring girls in his state to get the vaccine. As Perry launched his presidential bid, he called the order a mistake. "If I had it to do over again, I would have done it differently," he said during a debate.
Jena said there is also a "non-trivial number of pediatricians who are concerned about this issue," as well as surveys showing a significant chunk of parents have reservations about the HPV vaccine. There have been some studies that suggest those reservations about medical interventions and sexual activity could have a basis, but much of the research on HPV vaccines has been limited in scope or based on surveys, Jena said.
Given that, Jena said the "most appropriate approach" isn't just to dismiss such worries by parents and others. "But I think that's why you need scientific data to guide whether this is actually happening or not," he said.
"Suppose if someone were to offer me or a loved one a pill that would prevent me from ever getting colon cancer or lung cancer, or my wife, breast cancer. We would jump on that pill," Jena said. "We have available one of the first-ever medical therapies that could prevent someone from getting cancer, and yet less than 50 percent are taking up that therapy."