ATLANTA (AP) -- For the first time, an expensive vaccine aimed at preventing cervical cancer in women has proven successful at preventing a disease in men, according to a study released Thursday by the vaccine's maker. The disease is genital warts -- sexually transmitted, embarrassing and uncomfortable -- but not life-threatening.

Still, the results are expected to bolster a likely bid by the vaccine's manufacturer, Merck & Co. Inc., to begin marketing the vaccine to boys, experts said. Merck plans to ask the government for that approval later this year.

"This opens the door to a wonderful opportunity to prevent illness," said Anna Giuliano, a researcher at the H. Lee Moffitt Cancer Center in Tampa, Fla., who worked on the Merck study. The research results were being presented Thursday at a medical conference in Europe.

The focus was Merck's vaccine, Gardasil, which is given in three doses over six months and is priced at about $375. The vaccine targets the two types of HPV, or human papillomavirus, believed to be responsible for about 70 percent of cervical cancer cases, and two other types that cause most genital warts. HPV is spread through sex.

In 2006, the U.S. government licensed the vaccine for use in girls and women ages 9 to 26. Males can spread the virus, but the vaccine was not licensed for them because there was no evidence it prevented disease in men.

Though about 40 other countries have approved the vaccine for males, there still is no medical proof Gardasil prevents penile cancer or other HPV-associated cancers in men. There also is no evidence it prevents men from spreading HPV to women.

The new study involved about 4,000 males ages 16 to 26 in nearly 20 countries. Results showed the vaccine was 90 percent effective in preventing genital warts, with only 15 cases of persistent infection in the vaccinated group, compared to 101 cases in a group that was given a fake vaccine.

No serious side effects were reported. The research is continuing, but these results reflect how the men were doing about 30 months after the injections, on average.

The results are "very exciting," but it's not clear they will be enough to persuade many American families to get their teenage boys vaccinated, said Dr. Maura Gillison, an HPV researcher at Johns Hopkins University who was not involved in the Merck study.

She noted that only 1 in 4 girls have gotten the vaccine so far, despite compelling medical studies that indicate the shots prevent female cancers.

"When parents are sitting in a room discussing with a pediatrician whether to vaccinate their child against anything, they'd like to know what the potential benefit is. A parent might say, 'I'm not inclined to vaccinate my child to prevent a benign genital wart,'" she said.

Government officials have been awaiting this interim analysis from Merck. They are eager to see additional information that may come later on the vaccine's effect on precancerous lesions, said Dr. Lauri Markowitz, an HPV expert at the U.S. Centers for Disease Control and Prevention.

"It's obviously encouraging data, but the policy makers will be looking at a variety of different issues," including how cost-effective the shot would be if used in males, said Markowitz, a medical epidemiologist.

HPV causes at least 20,000 cases of cancer in the U.S. each year. Cervical cancer is the most common type, but about a quarter of cancers occur in men, including penile, anal and even head and neck cancers.

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