An experimental AIDS vaccine made from two failed products has protected people for the first time, reducing the rate of infection by about 30 percent, researchers said on Thursday.

Developers said they were now debating how to test the limited amounts of vaccine they have left to find out if there are ways to make it work better.

Scientists said they were unsure how or why the vaccines worked when used together in the trial, which took place in Thailand, and will study the volunteers to find out.

All agreed that a commercial product would be years away but the U.N. World Health Organization said the result created new hope that an effective vaccine would eventually be found.

The result of the study is a very important step for developing an AIDS vaccine, Thai health minister Withaya Kaewparadai told a news conference in Bangkok. It's the first time in the world that we have found a vaccine that can prevent HIV infection.

Activists and researchers alike were thrilled. The outcome is very exciting news and a significant scientific achievement, said Dr. Seth Berkley, chief executive officer of the International AIDS Vaccine Initiative, which was not involved in this study. It's the first demonstration that a candidate AIDS vaccine provides benefit in humans.

The vaccine is a combination of Sanofi-Pasteur's ALVAC canary pox/HIV vaccine and the failed HIV vaccine AIDSVAX, made by a San Francisco company called VaxGen and now owned by the nonprofit Global Solutions for Infectious Diseases.

The trial was sponsored by the U.S. government and conducted by the Thai Ministry of Public Health. It cut the risk of infection by 31.2 percent among 16,402 volunteers over three years.

We had 74 infections in the placebo group and 51 in the vaccine group, Dr. Jerome Kim, a U.S. Army colonel at the Walter Reed Army Institute of Research in Maryland, who helped lead the trial, said by telephone.

The results were a triumph for supporters, who went ahead with the giant trial despite criticism it was unethical or a waste of money because the vaccine was widely expected to have no effect.

The AIDS virus infects an estimated 33 million people globally and has killed 25 million since it was identified in the 1980s. It affects immune cells called T-cells.

Cocktails of drugs can control HIV but there is no cure. In 2007, Merck & Co ended a trial of its vaccine after it was found not to work, and in 2003, AIDSVAX used alone was found to offer no protection, either.

INSTILLING HOPE

The Geneva-based World Health Organization and the Joint United Nations Program on HIV/AIDS, or UNAIDS, stressed the effects of the vaccine in the Thailand test were modest.

However, these results have instilled new hope in the HIV vaccine research field and promise that a safe and highly effective HIV vaccine may become available for populations throughout the world who are most in need of such a vaccine, they said.

Sanofi shares rose as much as 1.6 percent in early trade in Paris but closed down slightly at 50 euros.

We see no commercial vaccine available for some time yet, but the prospect has finally been raised (after 30 years of trying) that an effective vaccine is possible, said Michael Leacock, an analyst at ABN AMRO research.

What is needed here is more in-depth analysis, Sanofi's Jim Tartaglia told a briefing. Dr. Donald Francis of Global Solutions for Infectious Diseases said the companies had limited amounts of vaccine left to test and would have to make more.

For me the biggest question is, how do you take what we know of this vaccine and give it greater protective power. Once you know that, then it makes sense to go to other parts of a population, Sanofi's Chris Viehbacher told Reuters.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which paid for most of the $105 million trial, said the team was confused because people who got the vaccine and who became infected anyway had just as much virus in their blood and just as much damage to their immune systems as HIV patients who went unvaccinated.

That meant the vaccine helped prevent infection but did nothing to affect the virus once it is in the body.

In addition, the immune responses that were generated by the vaccine should not, in theory, have protected anyone. Fauci said AIDS researchers may have to go back and see if they have been looking at the wrong things when checking the immune response to potential HIV vaccines.

Kim said the vaccine might not work in the people and places where HIV is most common -- in Africa, among men who have sex with men and among injecting drug users.

The vaccine was formulated specifically to work against two subtypes of the human immunodeficiency virus -- clade E, which circulates in Thailand and Southeast Asia, and clade B, common in the United States and Europe.

The volunteers in the trial got six shots over six months, four with ALVAC and two with AIDSVAX.

ALVAC is a genetically engineered canarypox virus that has spliced into it synthetic versions of three HIV genes. AIDSVAX is made using two versions of one HIV gene, one from the B subtype and one from the E subtype.

Thailand Medicine Aids Merck & Co.