Young women who get the recommended three doses of human papilloma virus (HPV) vaccine have fewer abnormal Pap tests than unvaccinated women and women who only get two doses, Canadian researchers say.

HPV vaccination is intended to prevent cervical cancer and abnormal cells that can lead to cancer, but how well it's working among North American women remains unclear.

"Both HPV vaccination and cervical cancer screening are important to further reduce the incidence, morbidity, and mortality of cervical cancer," senior study author Dr. Huiming Yang from Alberta Health Services in Calgary told Reuters Health by email.

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The province of Alberta implemented a school-based HPV vaccination program for girls in 2008 and expanded it to include boys in 2014. The HPV vaccination program provides three doses of the vaccine over a six-month period.

Yang and colleagues studied the impact on Pap test results eight years into this program.

The researchers analyzed data on 10,204 women, 56 percent of whom were unvaccinated and the rest had received at least one dose of HPV vaccine before having cervical cancer screening.

Overall, 14.5 percent of women had abnormal Pap tests and 85.5 percent had normal tests. Most abnormal tests, nearly 94 percent, were low-grade abnormalities, but the rest were the high-grade abnormalities that could progress to cervical cancer.

Less than 12 percent of women who had received at least three doses of the vaccine had abnormal Pap results, whereas 16 percent of unvaccinated women had abnormal tests. This translates into a 28 percent risk reduction with full HPV vaccination.

The difference was even greater - a 50 percent reduction in risk - when only high-grade abnormalities were included, according to the results published in CMAJ.

Surprisingly, incomplete HPV vaccination with two or fewer doses of the vaccine was not associated with a lower risk of having an abnormal Pap test.

"Our study shows that three doses HPV vaccination is very effective in reducing cervical cell abnormalities, particularly for high-grade lesions, but two does not appear to offer similar protection," Dr. Yang concluded. "It is important to complete all scheduled doses of vaccine."

"I found it interesting that young women in this study that received two doses of the quadrivalent vaccine had similar odds of having an abnormal Pap as those who had 0 doses," said Dr. Jacqueline M. Hirth from the University of Texas Medical Branch, Galveston, who has also studied the effect of HPV vaccination on cervical cancer screening among young women.

That result should be interpreted with caution, she told Reuters Health by email, since many of the young women in this sample were under 21 years of age and may not have been receiving routine Pap screenings.

"They may have actually gone to their providers for abnormal bleeding or other symptoms, which may have prompted them to receive Pap tests that would not have met the criteria for 'routine' screenings," Hirth said.

HPV vaccination in combination with cervical cancer screening according to guidelines is important for the prevention of cervical cancer, Hirth said, because the HPV vaccine does not protect against all high-risk HPV types.

She added that it's important for women to receive all three doses of the HPV vaccine and to undergo cervical cancer screening regardless of vaccination history in order to reduce their risk of developing HPV-related cancers.