That goes against worries on the part of some that getting the vaccine - which is supposed to ultimately help prevent cervical cancer - would encourage girls to become sexually active or engage in riskier sex than they otherwise would.

"Some parents have expressed it as a concern," said Saad Omer, an infectious diseases and vaccine researcher from Emory University in Atlanta who worked on the study.

"Parents can be reassured at least based on the evidence that young girls who receive HPV vaccines did not show increased signs (of) clinical outcomes of sexual activity," he told Reuters Health.

The vaccine, which has been recommended for 11- and 12-year-old girls in the U.S. since 2006, is still controversial, and only about half of girls start the series of shots.

One of the arguments against it has been that vaccination will make pre-teens feel a false sense of security when it comes to sex.

The Centers for Disease Control and Prevention now calls for boys to get vaccinated as well.

For the new study, Omer and his colleagues analyzed databases from Kaiser Permanente Georgia, a managed care organization covering the Atlanta area. Out of 1,398 girls who were 11 or 12 when they saw their doctors in 2006 and 2007, 493 got at least one dose of the HPV vaccine.

Based on records of their primary care visits, 107 of the girls included in the study were given a pregnancy test through 2010, and 55 were tested for Chlamydia.

Girls who did or didn't get the HPV vaccine were equally likely to be tested for both.

Two girls in each group got pregnant during the study. One girl who'd been vaccinated was diagnosed with Chlamydia, compared to three unvaccinated girls, according to findings published Monday in the journal Pediatrics.

Omer's team didn't have any data on how many of the girls were sexually active during the study period. The researchers also couldn't tell if the STI and pregnancy tests were part of standard clinical procedures or if girls were tested because they were sexually active or had symptoms.

Divya Patel, an ob-gyn researcher from the University of Michigan in Ann Arbor, called the study a "really good first stab" at looking at the behavioral effects of HPV vaccination.

"But I felt that it didn't convey the full picture," she added.

For example, it's possible girls as young as 11 and 12 aren't even told what the vaccine is for, or only hear about it in the context of cervical cancer.

"I'd be really surprised if kids this age are knowing that, ‘I got a vaccine for HPV and it's a sexually transmitted disease,'" Patel, who wasn't involved in the new study, told Reuters Health.

Looking at pregnancy tests from primary care doctors' offices is also limiting, she said, because girls might be more likely to go to school health clinics, or buy a test off the shelves, in those situations.

She said future studies like this one, but in different locations and in older girls, will be needed to help piece together what's going on.

The researchers agreed that there are other issues limiting the popularity of the HPV vaccine besides concerns about a spike in sexual risk-taking.

"I think that perception of risk (with HPV) is not quite there. People don't really consider themselves at risk, or they're not concerned," maybe because it's so common, Patel said.

"We need to be aware of how to prevent it, and vaccination is the best way to do that."

SOURCE: bit.ly/jsoh2P Pediatrics, online October 15, 2012.

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