A new University of Michigan study has found that administering the human papillomavirus vaccine based on a woman's risk factors could keep the vast majority of eligible women from getting the cancer-fighting vaccine.

The results support a federal recommendation that all females ages 11 to 26 should get the vaccine, the study's lead researcher said.

The study finds that using risk factors including sexual history would not only exclude about 80 percent of eligible women but also would likely vaccinate a large number of women already infected with at least one of the four HPV strains the vaccination helps protect against.

"We found that the more risk factors a woman had, the more likely they were infected with one of the four strains of HPV, but we couldn't find a specific threshold of risk factors that would predict which women would have HPV and which wouldn't," said lead researcher Dr. Amanda Dempsey.

"What we found was whether you used risk factors or didn't, either was very poor for selecting women who were at risk for HPV," she added. "There's really no way to use risk factors to sort out women into who would or wouldn't benefit from the vaccine."

The study was published Wednesday in the journal Vaccine by researchers at U of M's C.S. Mott Children's Hospital's Child Health Evaluation and Research Unit.

The study was an attempt to address issues raised by the vaccine's high cost and conflicting recommendations for which patients should be vaccinated.

The American Cancer Society advocates vaccinating all females younger than 18, and selectively vaccinating women ages 19 to 26 based on a discussion between the patient and her doctor about her sexual history. However, the U.S. Centers for Disease Control and Prevention's Advisory Committee on Immunizations Practices recommends universal vaccination for all females ages 11 to 26, regardless of sexual experience.

The HPV vaccine is the most expensive routinely recommended vaccine and is not fully covered by all insurance plans or by state and federal financing. So researchers set out to see if there was a way to target its use based on a woman's risk factors for contracting the virus, Dempsey said.

"It's very exciting for people in medicine because now there's a way you can potentially prevent cancer through vaccination," Dempsey said. "That's the positive side. On the less exciting side is … there are some concerns raised as to whether the price will serve as a major barrier to vaccination."

The researchers used data from the National Longitudinal Study of Adolescent Health to test the effectiveness of a targeted approach to vaccination. The study looked at 3,276 women, ages 19 to 24.

There are more than 100 strains of HPV; about 40 of those effect the genital area and about 15 of that 40 can cause cancer. It's estimated that within about three years of becoming sexually active, about half of all women will be infected with genital HPV, even those with only one partner, Dempsey said. Most people who are infected have no outward symptoms, though the virus can be detected during an annual pap smear.

The vaccine helps protect against two high-risk HPV strains responsible for about 70 percent of cancer cases and two low-risk strains responsible for more than 90 percent of genital warts cases.

The researchers made no personal recommendations, but Dempsey said their conclusions support the Advisory Committee on Immunizations Practices' guidelines. She said the American Cancer Society's recommendations to use a selective approach for women over 18 "isn't a reasonable goal to be able to achieve."