BANGKOK: For the first time, an experimental vaccine has prevented infection with the AIDS virus, a watershed event and a surprising result after recent failures led many scientists to wonder whether a vaccine would ever be found.

The vaccine cut the risk of a person becoming infected with HIV by more than 31 percent in trials conducted with the help of 16,000 volunteers in Thailand.

Even though the benefit was modest "it's the first evidence that we could have a safe and effective preventive vaccine," said Col. Jerome Kim in a telephone interview. Kim helped lead the study for the US Army, which sponsored it along with the National Institute of Allergy and Infectious Diseases.