The controversial human papillomavirus (HPV) vaccine Gardasil prevented external genital lesions in boys and young men -- mainly genital warts -- associated with the four virus strains targeted by the vaccine, researchers reported.
In an industry-sponsored multinational randomized clinical trial conducted among more than 4,000 boys and young men, the vaccine also prevented infection by the four HPV strains, according to Anna Giuliano of the H. Lee Moffitt Cancer Center and Research Institute in Tampa, Fla., and colleagues.
The HPV vaccine did not significantly affect the incidence of penile, perianal, or perineal intraepithelial cancer, although numbers were small, Giuliano and colleagues reported Feb. 3 in the New England Journal of Medicine.
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The vaccine has been approved to prevent cervical, vulvar, and vaginal cancer, as well as precancerous lesions caused by HPV types 6, 11, 16, and 18, in girls and women ages 9 through 26. It's also approved to prevent genital warts caused by types 6 and 11 in both males and females.
In December, 2010, on the recommendation of an advisory panel, the FDA expanded the indication to include anal cancer and anal intraepithelial neoplasia (AIN) grades 1, 2, and 3 caused by the four HPV vaccine types.
Outside experts greeted the current report with a mixture of enthusiasm and caution.
In an accompanying Perspective article, Jane Kim of Harvard School of Public Health, said the report is "cause to celebrate the extraordinary potential" of HPV vaccination to improve public health.
But Kim and others added that the expense of the vaccine -- about $400 for the complete series of three shots -- combined with concerns about the durability of its immune response gives it an uncertain future as a preventive measure in boys and men.
Giuliano and colleagues studied 4,065 healthy boys and men, ages 16 to 26, from 18 countries in a randomized, placebo-controlled, double-blind trial of the vaccine. The primary efficacy goal was to show that the vaccine reduced the incidence of external genital lesions related to HPV types 6, 11, 16, or 18.
The volunteers were analyzed on an intent-to-treat basis, all participants who got either vaccine or placebo, as well as per-protocol -- which included only those who got vaccine or placebo and were negative for 14 strains of HPV, including the four vaccine strains, at baseline. The analyses were meant to mimic the general population and boys before their first sexual encounter, respectively.
Volunteers had to have reported one to five sexual partners during their lifetime. Enrollment took place over a four-year period from Sept. 3, 2004, and Aug. 29, 2008, and the median follow-up was 2.9 years, Giuliano and colleagues reported.
But a major shortcoming of the trial is the limited follow-up, according to Dr. Diane Harper, of the University of Missouri Kansas City. In an e-mail to ABC News/MedPage Today, Harper said "duration of efficacy is not addressed, which is always, always, always the most important question" for public health purposes.
While the researchers reported that levels of anti-HPV antibodies peaked seven months after the last shot, Harper noted they gave no information on how quickly those levels tailed off. In trials among girls and women, she said, some women lost immunity to vaccine strains within five years.
The bottom line, she said, is that the vaccine is effective in males against genital warts, a non-life-threatening condition that is one-tenth as common as abnormal Pap smears among women -- but has yet to be shown to be effective against more serious disease.
On the other hand, the study is well done, rigorous, and adds important information to the discussion of whether to strengthen the current permissive recommendation to vaccinate boys, commented Dr. William Schaffner of Vanderbilt University School of Medicine in Nashville, Tenn.
However, the data by themselves are not enough to "move the needle" toward a recommendation for universal vaccination, Schaffner said in an e-mail to ABC News/MedPage Today.
Like most experts reached by ABC News/MedPage Today, he said the cost of the vaccine is high and must be outweighed by its benefits -- an analysis that still needs to be done. "The higher the cost of the vaccine, the more benefit has to be demonstrated," Schaffner said.