Test tubes of HIV-infected blood that has separated into white and red blood cells. (Christof Stache/Associated Press)

An experimental vaccine appears to cut the risk of becoming infected with HIV by 31 per cent.

The research by Thai and American scientists has not been peer-reviewed or published yet, but scientists worldwide have been watching the trial so closely and awaiting the results that the researchers announced their data analysis Thursday in Bangkok.

It's the first time a vaccine seems to have prevented HIV infections. The modest reduction was enough to encourage some researchers after 26 years of fruitless searching for a vaccine.

"It gives me cautious optimism about the possibility of improving this result," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which sponsored the study with the U.S. army.

Researchers tested a combination of vaccines in more than 16,000 men and women, ages 18 to 30, in Thailand who were HIV-negative when the trial began. They were followed for three years after vaccinations ended.

All participants were given condoms, counselling and treatment for any sexually transmitted infections they contracted during the trial and were tested every six months for HIV. Any participant who became infected was given free treatment with antiviral medicines.

Combination vaccine

According to the results, new HIV infections occurred in 51 of the 8,197 people given the vaccine and in 74 of the 8,198 people who received placebo shots. The people who received the vaccine had a 31 per cent lower risk of infection than those who didn't.

The goal of the study was to reduce the risk of HIV infection by 50 per cent. Ideally, vaccines should be at least 60 per cent effective, Fauci said.

The vaccine, referred to as a "prime-boost combo," is a combination two vaccines, ALVAC and AIDSVAX, which were not effective individually in earlier trials.

The trial was blind, meaning neither participants nor researchers knew who was getting what shot until the study ended.

It was hoped that the vaccine combo might help those who were infected during the course of the trial from getting full-blown AIDS, but the vaccine had no effect on the levels of HIV in the blood of those participants.

The trial was a "moderate success," said Dr. Walter Schlech, a professor of microbiology and immunology at Dalhousie University in Halifax and the Atlantic director of the Canadian HIV Clinical Trials Network, who said he was not overwhelmed by the results but said they were, nevertheless, a step forward.

Past vaccine trials failed so badly that some researchers were starting to despair about the prospect of a vaccine for HIV, he said.

Even with the trial's early success, it will be years and perhaps more than a decade before a vaccine is widely available, experts cautioned. Its also not yet known whether the vaccine combination will work against strains of HIV other than those contracted by the study participants.

UNAIDS estimates that every day, 7,500 people worldwide are newly infected with HIV. Two million died of AIDS in 2007, the UN agency said.

Details of the $105-million-US study will be presented at a vaccine conference in Paris in October.

Vaccine makers Sanofi Pasteur and Global Solutions for Infectious Diseases also sponsored the study.

With files from The Associated Press