Until recently, it seemed unimaginable: a vaccine that could prevent cancer? But last week the Food and Drug Administration approved a vaccine intended to do just that. Gardasil, manufactured by Merck, protects against two aggressive strains of the human papillomavirus (HPV) that lead to cervical cancer, the second most common cancer in women (250,000 deaths a year worldwide). In clinical trials involving about 21,000 women, the vaccine showed remarkable results: nearly 100 percent protection from HPV 16 and 18, which cause 70 percent of all cervical cancers. "This is the biggest breakthrough since the invention of the Pap test," says Dr. Mark Einstein, gynecologic oncologist at Montefiore Medical Center in New York. HPV is the most common sexually transmitted infection in the United States; 80 percent of sexually active women will be exposed to it, according to the Centers for Disease Control and Prevention. Often the virus is harmless, but some strains are not. Merck's vaccine also protects against 90 percent of genital warts in men and women caused by another HPV strain, and appears to prevent lesions that could lead to vaginal and vulvar cancers. The next challenge, officials say, is getting the vaccine out there. To be most effective, it must be administered to girls before they are sexually active. The CDC's Advisory Committee on Immunization Practices will meet this month to issue a recommendation on whether the vaccine should be given to all school-age females (it's been approved for those as young as 9)--and who will pay for it ($120 a dose, with three shots required). Then it will be up to states to determine if immunization is required for school. That's not a given. Many religious groups that advocate abstinence, like Focus on the Family, say they oppose mandatory HPV vaccinations.