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.

AP reports:

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.

Even though the benefit is modest, "it's the first evidence that we could have a safe and effective preventive vaccine," Col. Jerome Kim said in a telephone interview. 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 said he was surprised and very pleased by the outcome. "It gives me cautious optimism about the possibility of improving this result" and developing a more effective AIDS vaccine, Fauci said in a telephone interview. "This is something that we can do."

"Today marks an historic milestone," said Mitchell Warren, executive director of the AIDS Vaccine Advocacy Coalition, an international group that has worked toward developing a vaccine. "It will take time and resources to fully analyze and understand the data, but there is little doubt that this finding will energize and redirect the AIDS vaccine field," he said in a statement.

Even a marginally helpful vaccine could have a big impact. Every day, 7,500 people worldwide are newly infected with HIV; 2 million died of AIDS in 2007, the U.N. agency UNAIDS estimates. READ MORE