Merck licenses the Gardasil technology from CSL Ltd. of Australia. The vaccine was approved in Mexico last week. Merck has filed applications in five continents, including the large European Union market as well as Australia, Argentina and Brazil. The company is also trying to expand Gardasil's availability to developing countries.

The FDA says human papillomavirus is the most common sexually transmitted infection in the U.S. The Centers for Disease Control and Prevention estimates that about 6.2 million Americans become infected with genital HPV each year and that more than half of all sexually active men and women become infected at some time in their lives.

On average, there are 9,710 new cases of cervical cancer and 3,700 cervical-cancer deaths each year in the U.S. Worldwide, cervical cancer is the second most common cancer in women, with an estimated yearly toll of 470,000 new cases and 233,000 deaths.

Merck says it is important to vaccinate girls and women with Gardasil before they are exposed to HPV. "Adolescents are an important group to vaccinate against HPV. "One in four people ages 15 to 24 are infected with HPV," the company says.

The FDA was more blunt, saying the vaccine is only effective when given prior to infection. However, Merck says that if Gardasil is given to a woman infected with one type of HPV targeted by the vaccine, she still could be protected from the three other types covered by the vaccine.

Because Gardasil doesn't protect against less common types of HPV, the FDA says routine and regular pap tests "remain critically important to detect precancerous changes in the cervix to allow treatment before cervical cancer develops."