Vaccinating boys against the virus that causes cervical cancer and genital warts does not appear to be cost-effective, U.S. researchers said on Thursday.

Assuming all girls get the shot, adding boys to a national vaccination program may not be worth the expense, they said.

The U.S. Food and Drug Administration is weighing whether to approve Gardasil -- Merck & Co's vaccine against human papillomavirus -- for use in boys and men ages 9 to 26 to protect them from genital warts and prevent them from spreading the virus to their sexual partners.

What our results imply is for the resources expended, there may be better uses and other health interventions that would increase health gains in the population, Jane Kim of the Harvard School of Public Health, whose study appears in the British Medical Journal, said in a telephone interview.

Gardasil is approved for use in girls and young women to protect against certain strains of the virus.

And the U.S. Centers for Disease Control and Prevention recommends the shot for 11- and 12-year-old girls and for women 13 to 26 who have not been vaccinated.

The vaccine had global sales of $1.4 billion in 2008, with an additional $865 million received through a joint venture with Sanofi-Aventis.

But growth has slowed recently in markets where the initial group of eligible teenage girls has now been immunized.

The company is now seeking approval for use in boys and men. In clinical trials, the vaccine has been shown to offer a high degree of protection, Kim said.

Even though it might be beneficial, whether or not the benefits are worth the investment is what we sought to evaluate, she said.

She and colleague Sue Goldie used data from clinical trials, population studies and cost data to develop computer models to simulate the benefits of the vaccine over time.

They included the benefits of preventing cervical cancers, genital warts and a rare respiratory condition called recurrent respiratory papillomatosis that a pregnant woman with genital warts can pass to her baby.

To decide this, they calculated quality-adjusted life years, a figure that takes into account the impact of disease on quality of life.

They found using the vaccine in girls was well within the threshold for good value. But when they added boys into the equation, the benefits did not outweigh the costs.

Kim said the vaccine is new, so all the health benefits of giving the vaccine to boys may not be known.

And the study assumes that 75 percent of girls will get the vaccine and be protected from cervical cancer.

If coverage in girls ends up being low, then vaccinating boys became much more attractive, she said.

Kim said the findings do not suggest that the FDA should not approve the vaccine in boys, but that it might not be cost-effective to recommend routine vaccination in boys.

Merck & Co. Medicine