WASHINGTON (Reuters) - A vaccine designed to protect women and girls from cervical cancer caused by a wart virus may protect men, too, maker Merck and Co reported on Thursday.

The Gardasil vaccine was 90 percent effective in preventing lesions, mostly sexually transmitted warts, caused by the virus in men, Anna Giuliano of the H. Lee Moffitt Cancer Center & Research Institute in Tampa, Florida, and colleagues found.

It was about 45 percent effective in preventing infection with the four strains of HPV that it targets.

“We see 90.4 percent efficacy is reducing external genital lesions in males related to these four types of HPV -- 6, 11, 16, 18,” Giuliano said in a telephone interview.

“These are the only data evaluating efficacy of any HPV vaccine in preventing disease in males, and were presented for the first time this week at the European Research Organization on Genital Infection and Neoplasia International Multidisciplinary Conference,” Merck said in a statement.

The human papilloma virus, or HPV, is the most common sexually transmitted disease in the world. About 20 million Americans currently are infected with HPV, according to the U.S. Centers for Disease Control and Prevention.

It is the main cause of cervical cancer, which kills 3,870 women a year in the United States and 300,000 globally.

It can also cause other types of cancer, including anal and penis cancer as well as mouth and neck cancer. The CDC estimates that HPV caused 25,000 cases of cancer a year in the United States between 1998 and 2003.

Gardasil and its rival, GlaxoSmithKline’s Cervarix, are only approved for use in girls and women, but the companies are seeking new markets and some experts say it should be used in boys and men, to protect them and their future sexual partners.

Merck said it remains on track to submit a U.S. application by the end of the year for the use of Gardasil in males ages 9 to 26.

Giuliano’s team tested 4,065 young men ages 16 to 26, giving them either vaccine or placebo, and then checking them every six months. Evidence suggests the vaccine has to be given before a person is ever infected with HPV to be effective.

While it protected fewer than half the cases of HPV infection as detected in the blood, the vaccine appeared to prevent the development of genital warts and a precancerous

condition called penile/perineal/perianal intraepithelial neoplasia or PIN.

“This opens up some really important questions for further research,” Giuliano said. “The cancers in men, which are HPV related are really only now being understood.”

It will also be important to study whether vaccinating men protects female sexual partners, she said.