The vaccine against human papillomavirus does not cause riskier behaviour in teens, nor does it raise rates of sexually transmitted infections, US researchers say.
The findings by a team of experts at Harvard Medical School and the University of Southern California were published in JAMA Internal Medicine, a journal of the American Medical Association on Monday.
Health authorities recommend the vaccine, which was pioneered in Australia, for pre-teen and teenage boys and girls as a way to prevent HPV, which can go undetected and lead to cancers of the cervix, anus, penis, mouth and throat.
But just 38 per cent of girls aged 13-17 in the United States had received all three recommended doses in 2013, and even fewer boys were vaccinated, according to the US Centres for Disease Control and Prevention.
Vaccination rates have been slowly rising in the years since 2006, when the vaccine was introduced. In 2013, the most recent year in which officials have data, 57 per cent of girls and 35 per cent of boys began the HPV vaccine series with at least one of the three recommended shots, the CDC says.
But experts would like to see even higher rates of vaccination.
"Just as we do not wait until we have been in the sun for two hours to apply sunscreen, we should not wait until after an individual is sexually active to attempt to prevent HPV infection," wrote Robert Bednarczyk of Emory University, in an accompanying editorial.
In an effort to see if vaccination could be linked to higher STI infection rates, the study compared 21,000 girls who were vaccinated to 186,000 unvaccinated girls of the same age.
Both groups had the same insurance plans and lived in the same area of the United States.
Rates of sexually transmitted infections were measured four times per year, for the year prior to vaccination and the year after.
The girls who were vaccinated had slightly higher rates of STI than the unvaccinated girls, both before and after vaccination.
Researchers said they believe these higher rates were attributable to the higher likelihood of sexual activity among girls who are getting vaccinated, compared to unvaccinated girls.
However, both groups experienced an increase in rates of STIs as they grew older, and this increase occurred at the same pace, whether or not the girls had received the vaccine.
This "suggests that the girls' sexual behaviours were not altered in the least by the vaccine.
Any behaviours resulting in infections that did occur were independent of the vaccine," said the study.
If the HPV vaccine had caused an increase in risky sexual behaviour, researchers said they would have found a higher rate of STI among vaccinated girls after they received the vaccine, since it only protects against HPV and not any other diseases.
The study was funded by a grant from the US National Institutes of Health.
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