A vaccine to prevent HIV infection, the virus that leads to AIDS, has shown modest results for the first time, researchers have found, raising hopes that a disease that kills millions every year may someday be beaten.

In what is being called the world's largest HIV vaccine trial ever -- involving more than 16,000 participants in Thailand -- researchers found that people who received a series of inoculations of a prime vaccine and booster vaccine were 31 percent less likely to get HIV, compared with those on a placebo.

"Before this study, it was thought vaccine for HIV is not possible," Colonel Jerome Kim, who is the HIV vaccines product manager for the U.S. Army, told CNN.

HIV is the human immunodeficiency virus, which is the virus that causes AIDS -- acquired immune deficiency syndrome.

Kim emphasized that the level of effectiveness of the latest vaccine was modest, but given the failures of previous HIV vaccine trials, "yesterday we would have thought an HIV vaccine wasn't possible."

He called the results from the trial an important first step that will help researchers work toward a more effective vaccine.

Researchers have tried to prevent the spread of HIV since they discovered its cause in 1986. Previous vaccine trials failed to prevent infection. And during one trial, the vaccine seemed to boost the chance of being infected, which ended testing early.

The new study was conducted in Thailand, with more than 16,000 people between ages 18 and 30 participating. They were all HIV negative at the beginning of the trial.

Nearly 8,200 received a placebo and a similar number received a combination of six vaccines over six months. All were followed for three years.

Researchers found that those who received the vaccine combination were 31 percent less likely to contract HIV compared with those on a placebo.