The first vaccine to show any sign of preventing the spread of HIV has provided the most encouragement since the 1990s in the three-decades quest to stem the global AIDS epidemic. Results of a trial involving more than 16,000 adults in Thailand indicate the vaccine regimen was safe and reduced by 31% the chance of infection with the AIDS-causing human immunodeficiency virus, or HIV, according to the U.S. National Institutes of Health, which helped fund the study.