The current study involved 339 young women between the ages of 13 and 21 who received at least one dose of the HPV vaccine. Fewer than half of them were sexually experienced at the time of vaccination. Participants were asked (by questionnaire) about their sexual experience and their understanding of risk of sexual transmitted diseases (STDs) before, and again 2 and 6 months after, their first dose of the HPV vaccine.
Most of the young women correctly understood that the HPV vaccine did not change their risk of contracting STDs other than HPV. A few girls thought (incorrectly) that the HPV vaccine would reduce their risk of other STDs. However, these girls were not more likely to engage in sexual activity; in fact they were less likely to do so. Overall, there was no evidence that HPV vaccination leads to riskier sexual behaviors. Hopefully this study will convince parents who haven't had their daughters vaccinated yet because they thought that it might encourage promiscuity, to go ahead and allow their daughters to be vaccinated.
A word of caution. Six months is not a lot of time to assess changes in sexual activity in girls as young as 13. After all, most girls that age are not yet sexually active. It would be worth seeing if there are any changes in attitudes or sexual behavior further out; say, 5-10 years from now.