Attendees of the upcoming 15th International AIDS Conference in Bangkok will hear the results to date from the world's largest HIV vaccine trial, Thai officials announced this week. The U.S.-sponsored Phase III clinical trial will include 16,000 HIV-negative volunteers in Thailand to test whether a two-part vaccine made by VaxGen and Aventis Pasteur is effective at preventing HIV infections. The study is testing the "prime-boost" vaccination concept, in which a first vaccine--VaxGen's AIDSVAX B/E--is used to prime the immune system to fight invading HIV and Aventis Pasteur's Alvac is used to boost those immune system defenses against the virus. More than 6,000 people are already signed up for the trial, with more than 4,000 already vaccinated, Thai health officials say. The conference is July 11-16.