An experimental Aids vaccine made using two older vaccines protected volunteers, lowering the risk of infection by about a third, US and Thai researchers reported on Thursday.

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.

It lowered the risk of HIV infection by 32 percent among 16,000 heterosexual Thai volunteers who had no special risk of Aids infection, the US and Thai government researchers said.

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

The result, almost completely unexpected, puzzled researchers, who say they cannot figure out why the vaccine combination is working.

It is also a triumph for its supporters, who went ahead with the giant trial of 16,000 volunteers despite critics who said it was unethical or a waste of money because the vaccine was so widely expected to have no effect at all.

'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 US National Institute of Allergy and Infectious Diseases, which helped pay for the study.

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

Further muddying the waters -- 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.

This means the vaccine helps to prevent infection but does nothing to affect the virus once it is in the body.

'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 stressed that the vaccine may 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 tested in Thailand and it is really specific for the strains that are circulating in Thailand now,' Kim said.

Fauci said the Thai findings did not suggest whether any of these other dozens of other HIV vaccine trials should be dropped, or pursued more actively.

'I know people are going to be asking the question, should we be using it right away?' Fauci said.

But he and Kim said the researchers need to study the data, to see why it protected some people. Both also 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, which is common in the United States and Europe.

The 16,000 volunteers in the trial got six immunizations 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.

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<