MARILYNN MARCHIONEBANGKOK (AP) ¿ For the first time, an experimental vaccine has prevented infection with the AIDS virus, a watershed event in the deadly epidemic and a surprising result. Recent failures led many scientists to think such a vaccine might never be possible. The World Health Organization and the U.N. agency UNAIDS said the results "instilled new hope" in the field of HIV vaccine research, although researchers say it likely is many years before a vaccine might be available. The vaccine ¿ a combination of two previously unsuccessful vaccines ¿ cut the risk of becoming infected with HIV by more than 31 percent in the world's largest AIDS vaccine trial of more than 16,000 volunteers in Thailand, researchers announced Thursday in Bangkok. Even though the benefit is modest, "it's the first evidence that we could have a safe and effective preventive vaccine," Col. Jerome Kim told The Associated Press. He helped lead the study for the U.S. Army, which sponsored it with the National Institute of Allergy and Infectious Diseases. The institute's director, Dr. Anthony Fauci, warned that this is "not the end of the road," but he said he was surprised and very pleased by the outcome.