Study co-author Robert Bednarczyk, a clinical investigator with the Kaiser Permanente Center for Health Research–Southeast, said in an interview with The Huffington Post that researchers found "no difference" in sexual activity when comparing girls who were vaccinated to those who were not.
In findings published in the journal Pediatrics on Monday, investigators combed the electronic medical records of nearly 1,400 girls who were between the ages 11 and 12 from 2006 to 2007 -- the year following the Food and Drug Administration's approval of the first vaccine. Nearly 500 of the girls had received it; 900 had not.
Unlike previous studies that have largely relied on self-reported sexual activity, the new study measured clinical signs of sexual activity over several years, like whether the girls had been tested for a sexually transmitted infection, received contraceptive counseling or become pregnant.
"What we've done here is take a more objective approach and use clinical data to look at outcomes that are related to sexual activity and compare them," Bednarczyk said.
Vaccination was not linked to any increased sexual activity, according to the measures used by the researchers.
"This article supports what several years of data have shown again and again," said Gypsyamber D'Souza, an assistant professor at the cancer prevention and control program at Johns Hopkins Bloomberg School of Public Health, who did not work on the study. "There are no differences in sexual behavior in those getting the vaccine compared those who have not."
Recent reports suggest that many girls are not vaccinated in part because of fears about increased sexual activity. One such report, a 2011 paper in the journal Sexually Transmitted Infection, found that 16 percent of parents in areas of North Carolina felt that teenage girls who were vaccinated might be more likely to have sex.
"The reality is that [the rate] of HPV vaccination is substantially lower than other recommended adolescent vaccines," said Dr. Matthew Davis, an associate professor of adult medicine and pediatrics at the University of Michigan's C.S. Mott Children's Hospital.
In 2011, some 80 percent of teens were vaccinated against tetanus, an infection of the nervous system, and 70 percent against meningitis, an infection of the membranes around the brain and spinal cord, according to Centers for Disease Control and Prevention estimates. But only 35 percent of teenage girls received the full three doses of the HPV vaccine, which has been recommended for all girls age 11 or 12 by the CDC since 2006. (The recommendation also extends to women up to age 26 who were not vaccinated when they were younger.) The vaccine protects against infection by certain strains of HPV, about 30 types of which can lead to cervical cancer in women.
Davis recently led a poll using a national sample that found that 40 percent of parents have moral or ethical concerns about the HPV vaccination, while 36 percent said they felt pre-teen girls were too young to get it.
"Those [rates], to me, indicate that there remain concerns that adolescent's choices will be influenced by this vaccination," he said.
But D'Souza argued that other factors may be more influential.
"Parents have concerns about the cost of the vaccine, the utility of the vaccine, its side effects and whether it's truly necessary," she said, also noting that the way doctors describe the vaccine to patients can impact patients' perception of it. Cost is a major issue, especially for college-age women, D'Souza added. Though most private insurers now cover the HPV vaccine, and the federally-funded Vaccines for Children program provides free shots for eligible individuals, many young women do not know that.
Davis said parents' fears about risky sexual behavior are a powerful deterrent, but expressed hope that the new findings could impact their thinking.
"It's hard to know what impact a study like this can have on parental opinions," he said. "We do know that physicians and nurses can be very trusted sources of information about vaccines for parents. Therefore, if health care providers hear this news and can themselves reassure parents that teenagers' sexual behaviors are not likely to change based on this vaccination, that may reassure parents."
"I would say the reason why HPV has been so controversial is precisely because it goes right to the heart of sexual behavior," Davis said. "HPV is spread sexually, and not through other means."