Women for the first time have a vaccine to protect them against cervical cancer. The Food and Drug Administration approved use of the vaccine, Gardasil, for use in girls and women ages 9 to 26. It works by preventing infection by four strains of the human papillomavirus, or HPV, the most prevalent sexually transmitted disease. This cancer kills 3,700 women each year in the United States and hundreds of thousands more worldwide. Gardasil, manufactured by Merck & Co. Inc., protects against the two types of HPV responsible for about 70 percent of cervical cancer cases. The vaccine also blocks infection by two other strains responsible for 90 percent of genital wart cases. Clinical trials showed Gardasil prevented 100 percent of cervical cancer related to the two HPV strains in women who had not been previously infected, Merck said. Merck is expected to market Gardasil as a cancer, rather than an STD, vaccine. It remains unclear how widespread will be the use of the three-shot series, in part because of its $360 list price. Conservative opposition to making the vaccine mandatory for school attendance may also curb its adoption. The target age for receiving Gardasil is low because the vaccine works best when given to girls before they begin having sex and run the risk of HPV infection. The vaccine may not protect people already infected and may increase their risk of the kind of lesions that can lead to cervical cancer, the FDA has said.