WASHINGTON — The first vaccine to protect against most cervical cancer won federal approval Thursday. The vaccine Gardasil, approved for use in girls and women ages 9 to 26, prevents infection by four strains of the human papillomavirus, or HPV, Merck & Co. (MRK) Inc. said. The virus is the most prevalent sexually transmitted disease. Gardasil 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. 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 estimated cost of $300 to $500. 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 […]