Young women immunized against human papilloma virus were significantly less likely to have potentially cancerous cervical abnormalities compared to those who didn't receive the HPV vaccine, a study of more than 10,000 subjects has found.

In a study published Monday in the Canadian Medical Association Journal, Alberta researchers found that young women who were immunized as girls against the sexually transmitted virus as part of a school-based inoculation program had a 50 per cent lower risk of having cervical-cell anomalies than their unvaccinated counterparts.

Among vaccinated women in the study, Pap screening showed 11.8 per cent had abnormal cervical cells, compared to 16.1 per cent of those who hadn't been immunized against HPV.

"Our study demonstrated that HPV vaccinations are highly effective in reducing cervical cell abnormalities, particularly against the high-grade lesions which have the potential to become cancerous later in life," said principal investigator Dr. Huiming Yang, medical director of screening programs at Alberta Health Services.

The study also found that three doses of the quadrivalent HPV vaccine — which protects against four subtypes of the virus, two of which are responsible for 70 per cent of cervical cancer cases — appeared to offer better protection than two doses, Yang said from Calgary.

There are two HPV vaccines licensed in Canada: Gardasil, which is designed to protect against four virus subtypes that cause cervical cancer and genital warts; and Cervarix, a bivalent vaccine against the two subtypes that cause the majority of cervical malignancies.

Both have been licensed to be given in three separate doses to females aged nine to 26. Gardasil has also been recommended for use in males of the same age, while Cervarix is currently not approved for boys and young men.

However, the National Advisory Committee on Immunization (NACI) now recommends that both vaccines may be administered in two separate doses, six to 12 months apart, for those aged nine to 14.

Without vaccination, it's estimated that 75 per cent of sexually active Canadians will be infected with HPV at some point in their lives, NACI says.

Frequency of screening?

The CMAJ study also found that females who had been vaccinated against HPV were more likely than non-immunized females to undergo Pap tests to screen for cervical cancer.

Yang isn't sure why this is the case, but he speculated that parents who consented to have their daughters vaccinated under the Grade 9 program might also have taught them about the risks of HPV infection, including the increased danger of developing cervical cancer.

"So maybe that prompted them to have more discussion with children at that age, and later on [the girls] would be more likely to be tested," he said, noting that little research has been done on that issue.

The study's results suggest HPV vaccination and screening are both important for reducing the risk of cervical cancer, "even among those that are vaccinated, because the risk is still there — it's only lower," said Yang.

"We know the people who are vaccinated are at a lower risk of cervical cancer, so we may [want to] tailor our cancer screening program to those people who are vaccinated versus unvaccinated, because the risk levels are different now.

"So the frequency of screening may need to be reconsidered and also the intervals [between shots] may need to be looked at as well."

However, Yang said more research is needed before changes are made regarding cervical cancer screening and the optimal dosage and timing of vaccinations.

An estimated 1,500 Canadian women were diagnosed with cervical cancer in 2015 and about 380 died from the disease, the Canadian Cancer Society says.