A Thai official (R) walks past an anti-AIDS campaign poster displayed at the Thai Ministry of Public Health in Bangkok on September 24, 2009. An experimental AIDS vaccine has for the first time cut the risk of infection in a 'breakthrough' in the quarter-century battle against the deadly epidemic, researchers said. (Getty Images)

U.S. Ambassador to Thailand Eric John, left, shakes hand with Thailand Minister of Public Health Witthaya Kaewparadai after the press conference in Bangkok on Thursday, Sept. 24. 2009. Researchers in Thailand reported 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. The vaccine 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. (AP)