Girls who got vaccinated against the sexually transmitted infection human papillomavirus (HPV) were more likely than their unvaccinated peers to become infected with a sexually transmitted disease, but the vaccine was definitely not to blame for their risky sexual behaviour, according to a new study of more than 200,000 American teens. Even for girls who didn't get the HPV vaccine, the risk of being diagnosed with an STI increased with age, the study authors found. By comparing the changes in infection rates for both groups of girls, the researchers were able to isolate the effect of the vaccine - and they found that it was non-existent. "We found no evidence that HPV vaccination leads to higher rates of STIs," they wrote in a study published on Monday in JAMA Internal Medicine . The vaccine protects adolescent girls and women against the strains of HPV that cause most cases of cervical cancer. HPV can also cause vaginal, vulvar, anal, penile and oropharynx cancers, which is why the vaccines are recommended for adolescent boys as well as girls. Two versions of the vaccine - Gardasil and Cervarix - have been endorsed by the Centres for Disease Control and Prevention, though Gardasil fights more strains of HPV and therefore protects against a wider range of cancers, as well as genital warts. Both vaccines require three doses to take full effect, and experts recommend that boys and girls begin the series when they are 11 or 12 years old. That way, they are fully protected before they become sexually active. However, many parents - and even some doctors - have been reluctant to vaccinate children before they become teenagers due to fears that it may encourage them to start having sex. Even in cases where teenagers are already sexually active, the vaccine might make them less inclined to use condoms or take other safe-sex precautions if they feel their bodies are already armed against STIs, some worry. Attitudes like these are largely responsible for the low vaccination rates in the US, according to nationwide surveys. In 2013, only 57 per cent of girls between 13 and 17 had even started the HPV vaccine regimen, and only 38 per cent got all three doses. Various small studies have found no link between vaccination and sexual activity, but the study subjects didn't reflect the nation as a whole. So a group of researchers from Harvard and the University of Southern California examined six years' worth of insurance claims from around the country to see if they could get more definitive answers. The researchers found that the 21,610 young women who were vaccinated were more likely to have a sexually transmitted infection than the 186,501 young women who weren't. For the researchers, the real question was whether the infection rate grew faster among girls who were vaccinated than among girls who weren't. After crunching the numbers, they determined that the increases for both groups were essentially the same.