An experimental AIDS vaccine made from two older versions 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 better. Scientists said they are unsure how or why the vaccines worked and will study the volunteers in the trial to find out.

Both agreed the vaccine's immediate usefulness may be limited and a commercial vaccine would be years away.

The result of the study is a very important step for developing an AIDS vaccine, Thai Health Minister Withaya Kaewparadai said. It's the first time in the world that we have found a vaccine that can prevent HIV infection.

The vaccine is a combination of Sanofi-Pasteur's ALVAC canary pox 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. Officials from the two countries told a news conference in Bangkok the risk of infection had been cut by 31.2 percent among 16,402 volunteers.


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.

Myself, like others, did not think there was a very high chance that this would give any degree of efficacy, said Dr. Anthony Fauci of the U.S. National Institute of Allergy and Infectious Diseases, which paid for most of the $105 million study.

But nonetheless, we went ahead with the trial and it was controversial to go ahead with it.

Sanofi shares rose as much as 1.6 percent in early trade in Paris but gave up the gains to fall 0.24 percent to 50.75 euros by 0904 GMT, while the DJ health index was down 0.57 percent.

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.

Fauci 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.

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.

Although the level of protection that we saw was clearly modest, the study is a major scientific advance, Kim said.

It is the first evidence that the development of a safe and effective vaccine is possible. Although we don't have all the answers now, it does have important implications for the future of HIV vaccine design.

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.

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

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

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 the virus 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.