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Researchers in Bangkok announced Thursday that the vaccine lowered the risk of becoming infected with HIV by 31 percent.

Scientists are calling the benefit modest. The key development is not the effectiveness of this specific vaccine, but rather the fact that a vaccine can be produced.

According to researchers, 16,000 volunteers between the ages of 18 and 30 took part in the study – making it the world's largest AIDS vaccine trial.

Half were given the two vaccines, while half were given a placebo. After three years, 74 people in the group that didn't get the vaccine wound up HIV-positive, compared to 51 in the vaccinated group, a difference scientists say is significant.

Top researchers in New York and around the world are calling it a major advance after more than 25 years of HIV and AIDS science, but say it still has to be taken in context.

"Most vaccines we use today are effective in 80 percent or more of people and that is how we judge whether a vaccine is reasonable, if it is a good idea to offer to people," says Dr. Roy Gulick of Weill Cornell Medical College. "Big caveat about this study, the effectiveness was just over 30 percent, so that's low. But the fact is it is more than zero percent, which every study that came before it effectively showed no activity towards preventing HIV infection."

Researchers are still trying to figure out how the vaccine combo actually worked.

The study used strains of HIV common in Thailand and scientists stress the vaccine may not work against other strains in the United States, Africa, or elsewhere.

Latest figures show 7,500 people worldwide are newly infected with HIV each day.

Health experts say some of the best ways to prevent the spread of HIV include abstinence, being faithful to one's partner and using condoms in sexual intercourse.