Most people are exposed to HPV within five to 10 years of their first sexual experience. Thus, the vaccine was approved for girls to prevent infections that might occur between ages 15 and 25. The study, by researchers at the National Institute of Cancer in Bogota, Colombia, and an international group of collaborators, examined the vaccine's effectiveness in almost 4,000 women with no history of genital warts or cervical disease. The women received either the HPV vaccine or a placebo. After two years, researchers found that four cases of infection or disease had occurred in the vaccine group (91 percent effectiveness) compared with 41 in the placebo group. Researchers also found that, among women who were not completely vaccinated (the vaccine is a three-shot regimen) or who had pre-existing HPV infection, the vaccine was only 31 percent effective against all four strains of HPV. Data on how the women fared after four years will become available later this year, according to Merck, the manufacturer of Gardasil. Based on that data, the company is expected to ask the Food and Drug Administration to approve the vaccine for women ages 25 to 45, thus opening the door to insurance reimbursement. The vaccine costs about $500. An HPV vaccine made by GlaxoSmithKline, Cervarix, is under review by the FDA. Cervarix has been show in studies to protect women up to age 55. The HPV vaccine might become available to boys, too. A study presented in November at a medical meeting in Europe found that Gardasil helped prevent genital warts in boys and men. Genital warts can lead to anal and penile cancer in men. Merck has requested approval of the vaccine for boys ages 9 to 26. 