AIDS vaccine researchers say they have some new clues to help focus their search for a safe and effective vaccine against HIV.

At the AIDS Vaccine conference in Bangkok on Tuesday, scientists announced an update to a Thai trial of a "modestly effective" experimental AIDS vaccine.

At the end of the initial 3½-year study, the vaccine prevented infection in about 30 per cent of the 16,000 Thai volunteers who received it compared with a placebo.

The new findings shed light on how the vaccine worked by identifying how the immune system responded to it.

Scientists have also discovered a part of HIV that may be vulnerable to a vaccine, which means scientists now have some idea of how the vaccine worked, and how to do more focused research.

The new AIDS vaccine findings could make it possible in the future to conduct vaccine trials with fewer people. ((Apichart Weerawong/Associated Press)) "It might be possible in the future to do vaccine trials with many fewer people much more rapidly so we could see progress occurring a lot more rapidly," said U.S. army Col. Jerome Kim, one of the leaders of the Thai trial.

Stephen Kent, a vaccine researcher at the University of Melbourne, described the antibodies produced by the vaccine as "friends with benefits."

"They don't necessarily prevent infection of cells but it allows them to kill infected cells," said Kent. "So it may be that that really helps protection."

More vaccine trials planned

The findings, along with the development of a test for the antibody response, may focus and speed up the vaccine research.

Scientists want to take the findings back to the lab or clinic to try to improve the effectiveness of the Thai vaccine. Results suggested that protection against HIV appeared highest at six to 12 months, which investigators hope to sustain or boost.

But the vaccine and the immune response it created were specific to the type of HIV in Thailand, and the vaccine formulation used there, scientists cautioned.

The findings suggested the vaccine had no effect on the amount of virus in the blood of those who became infected with HIV. Vaccination did seem to be associated with lower amounts of virus in genital fluids.

New trials will begin this fall in Thailand to test a booster, and a similar vaccine trial is being planned in South Africa to try to replicate the Thai results.

Despite the excitement, scientists agreed that before an HIV vaccine can be licensed for use, it will have to be more broadly effective, which is years away.

The research was funded by the U.S. Army Medical Research and Material Command, the U.S. National Institutes of Allergy and Infectious Diseases' AIDS division, and the Bill & Melinda Gates Foundation.

The conference runs through Thursday.