Oct. 16 (Bloomberg) -- Merck & Co.’s Gardasil, a vaccine used to prevent cervical cancer in women, won U.S. regulatory approval for preventing genital warts in boys.

The U.S. Food and Drug Administration cleared the vaccine for use in males ages 9 to 26, Merck said today in a statement.

Gardasil protects against a sexually transmitted infection called human papillomavirus, or HPV, that can lead to cervical cancer in women and genital warts and cancer of the penis and anus in men. Gardasil, approved for females ages 9 to 26, is given mostly to school-age girls as a U.S.-recommended routine vaccination. Expanding the shot’s use could revive sales, which declined 5 percent last year, analysts have said.

“This is an important milestone, because the use of Gardasil can now help protect boys and girls and young men and women from certain diseases caused by this common virus,” said Richard Haupt, executive director of Merck Research Laboratories, in the company’s statement.

Approval in boys could add as much as $200 million and $300 million in annual sales, said Leerink Swann & Co. analyst Seamus Fernandez in a research report last month. Gardasil generated revenue of $1.4 billion last year.

Merck, based in Whitehouse Station, New Jersey, fell 6 cents, or less than 1 percent, to $33.24 at 1:26 p.m. in New York Stock Exchange composite trading.

Difficult Market

Selling Gardasil for boys could be a struggle for Merck because the public-health benefit may not outweigh the expense, Fernandez said. It would cost more than $100,000 to vaccinate enough boys to get one year of additional life compared with less than $50,000 for girls, according to a study by Harvard University researchers presented in June to the U.S. Centers for Disease Control and Prevention. Merck’s studies show it would cost $50,000 for both boys and girls. Merck’s study used a cost of $400 per vaccine with 100 percent protection.

Researchers used a measure called quality adjusted life years, or QALYs, that evaluate years of life saved as well as assigning a fraction of a year for years spent without certain diseases.

Merck will expand a patient rebate and dose replacement program to help cover the cost of the vaccine for 19- to 26- year-old men without health insurance and those with private insurance with partial or no coverage for the shots, according to the company’s statement.

Company Study

In a Merck-funded study released last year, researchers gave 4,065 boys and men ages 16 to 26 the vaccine or a placebo, then tracked them for signs of infection with HPV. After about 30 months, three men getting Gardasil developed genital warts and none had pre- cancerous growths linked to the HPV virus, compared with 28 cases of warts and three pre-cancerous lesions in the placebo group.

Gardasil, which is given in three shots over a six- month period, protects against infection caused by HPV types 6, 11, 16 and 18 -- four of the 40 types of the virus found in the genital area. More than 1 million cases of genital lesions, which can lead to cancer, occur in men and women in the U.S. each year, and 30 million cases occur worldwide, according to Merck.

While 20 million Americans are infected with HPV, most will be able to fight off the infection naturally. About 1 percent of sexually active men in the U.S. will develop genital warts from HPV, the CDC said. Gardasil is already approved for males in 40 countries worldwide.

To contact the reporter responsible for this story: Shannon Pettypiece in New York at spettypiece@bloomberg.net.