Vaccinating young girls against the sexually transmitted human papillomavirus (HPV) doesn’t lead to an increase in risky sexual activity, a new study shows. 

Teen girls in the study who were vaccinated against HPV were no more likely to become pregnant or contract another sexually transmitted infection (STI) than girls who were not vaccinated, the researchers found.

The U.S. Centers for Disease Control and Prevention recommends that all boys and girls receive three HPV shots, which protects against multiple strains of the virus, which can cause cervical, anal, penile and throat cancer.

There has only been one other study of the potential association between the HPV vaccine and risky sexual behavior, which was much smaller and also found no relationship, the authors write (see Reuters story of October 15, 2012 here: reut.rs/1fDWl0Z).

“Our study was almost 200 times larger than the previous study and found no evidence of an increase in risk,” said lead author Leah M. Smith of McGill University in Montreal in an email.

The results, she added, “should help put any remaining uncertainty on this issue to rest.”

Beginning in 2007, all 8th grade girls in Ontario, around age 13, were offered all three doses of the HPV vaccine. Only half of the eligible girls did receive all three doses.

Using health databases in Ontario, the researchers studied more than 128,000 girls, half of whom were eligible for the vaccine in school. The other half had gone through grade 8 before the vaccine was offered.

In grades 10 through 12, more than 10,000 of the girls became pregnant, and 6,000 contracted an STI.

But the girls who had been vaccinated were no more or less likely to experience pregnancy or an STI than those who had not been vaccinated, the researchers reported in the Canadian Medical Association Journal.

“The few other studies on HPV vaccination and sexual behavior have focused on perceptions of changes in sexual behavior following vaccination, rather than actual behavior, or have relied on self-reports of sexual behavior, which are notoriously problematic to study because they are vulnerable to the recall bias, response bias, and social desirability bias,” Smith said.

But in terms of actual sexual behavior, sexual health interventions consistently do not lead to increases in risky sexual behaviors, she said.

Concerns about that question have led some parents to choose not to have their children vaccinated, she said.

“It is also one of the main reasons why some Catholic school boards in Canada initially refused to be part of the country’s publicly funded, school-based HPV vaccination program, and it is a reason why some religious organizations have spoken out against the vaccine,” Smith said.

Similar concerns in the U.S. may be less of a barrier

“At least two recent reports in the U.S. indicate that concern about increased risky sexual behavior is not a main reason to not get vaccinated,” said Jane J. Kim, citing two articles identifying financial concerns and parental need for more information as major barriers to vaccination.

Kim, an associate professor of Health Decision Science at the Harvard School of Public Health in Boston, was not part of the new study.

The most common side effect of the vaccination is pain at the injection site, fainting or dizziness, but no fatalities or disease have been causally linked to either of the two HPV vaccines, Gardasil or Cervarix, Kim said.

Fewer than 60 percent of young people in the U.S. get the first shot in the series, and fewer than 40 percent receive all three, she said.

“Other studies have consistently shown that sexual health interventions do not increase risky sexual behavior, and some may even promote safer sexual behaviors,” Smith said. “This includes interventions aimed at improving condom access in schools and providing sexual health education to teens.”