By Kathryn Doyle

(Reuters Health) – Counter to the fears of some parents (and a few doctors), teen girls who are vaccinated against the sexually transmitted human papillomavirus (HPV) do not go on to catch more sexually transmitted diseases than their peers, according to a large new study.

Some parents have been hesitant to have their teens vaccinated for fear that they would be emboldened to have more, and riskier, sex – the kind that can put them at risk for sexually transmitted diseases (STDs).

The HPV vaccine has been available in the U.S. since 2006 and is recommended for all preteen boys and girls at age 11 or 12, so they have time to develop an immune response before becoming sexually active. But by 2013, less than 40 percent of U.S. girls age 13 to 17 had received all three doses of the vaccine, the authors of the new study point out.

Using the rate of STDs as an indicator of risky sexual behavior, there doesn’t appear to be a connection with the vaccine, researchers say.

“The vast majority of parents don’t think this is an issue,” said Dr. Anupam B. Jena, who’s with the Department of Health Care Policy at Harvard Medical School in Boston. “But 10 to 20 percent of parents and at least some pediatricians think that this could be an issue.”

The recommended three doses of the vaccine protect against the two strains of HPV responsible for most cervical, anal and vaginal cancers. But even with the most recent data, less than half of teens have gotten the full three-shot series, Jena told Reuters Health by phone.

He and his co-authors analyzed insurance data on more than 200,000 teen girls from 2005 to 2010. Less than three percent had received the vaccine by the end of 2006, which increased to more than 25 percent by 2010.

In the year before they were vaccinated, about four of every 1,000 girls experienced a sexually transmitted disease (STD), such as Chlamydia, gonorrhea, herpes, human immunodeficiency virus or syphilis. Among girls who weren’t vaccinated, about three of every 1,000 had contracted one of these infections in the past year.

But over the following year, STDs became more common for both groups of girls, rising to more than six of every 1,000 vaccinated teens and more than four of every 1,000 unvaccinated teens, according to results in JAMA Internal Medicine.

Because rates of these infections increased in both groups as the girls got older, the HPV vaccine was not tied to any extra increase in STDs, the authors write. Those who were vaccinated may have started having sex earlier, which led to their parents or doctors recommending the vaccine, and which would explain the higher STD rate before and after the shots, they write.

Quite a bit of research has been done on the potential ‘behavioral compensation’ that may follow sex education, free condom programs, or other safe-sex interventions, and it seems from most of the studies like these efforts do not lead to riskier sexual behavior, Jena said.

Still, “it’s not an unreasonable concern for parents or policymakers to have,” he said. “That’s why you need studies like this.”

Hopefully, the results of this very large study of U.S. girls will dispel some of those concerns, he said.

Some people have been uneasy with the idea of recommending an STD vaccine for kids as young as 11 or 12, according to Robert A. Bednarczyk of Emory University in Atlanta, who wrote a commentary in the journal.

“They mention not feeling comfortable talking about sex with an 11 year old,” Bednarczyk told Reuters Health by phone. “But if you’re giving a tetanus and diphtheria shot, are you going into extended discussions about transmission?”

Eleven and 12 year olds are already coming to the doctor for a round of routine vaccinations including a tetanus-diphtheria-pertussis (Tdap) booster and a vaccine against bacterial meningitis, so it makes sense to start the HPV series at the same time, he said.

“If you give the vaccine at an earlier age it provides a higher level of antibodies,” Bednarczyk said.

Adolescents are going to be sexually active, so arming them with education and vaccines is important, he said.

SOURCES: http://bit.ly/1A3gEDP and http://bit.ly/1zMp4w2 JAMA Internal Medicine, online February 9, 2014.