A new AIDS vaccine tested on more than 16,000 volunteers in Thailand has protected a significant minority against infection, the first time any vaccine against the disease has even partly succeeded in a clinical trial.

Scientists said they were delighted but puzzled by the result. The vaccine — a combination of two genetically engineered vaccines, neither of which had worked before in humans — protected too few people to be declared an unqualified success. And the researchers do not know why it worked.

“I don’t want to use a word like ‘breakthrough’, but I don’t think there’s any doubt that this is a very important result,” said Dr Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, which is one of the trial’s backers.

“For more than 20 years now, vaccine trials have essentially been failures,” he went on. “Now it’s like we were groping down an unlit path, and a door has been opened. We can start asking some very important questions.”

Results of the trial of the vaccine, known as RV 144, were released at 11.30 am India time Thursday in Thailand by the partners that ran the trial, by far the largest of an AIDS vaccine: the US Army, the Thai Ministry of Public Health, Dr Fauci’s institute, and the patent-holders in the two parts of the vaccine, Sanofi-Pasteur and Global Solutions for Infectious Diseases.

Col Jerome H. Kim, a physician who is manager of the US Army’s HIV vaccine programme, said half the 16,402 volunteers were given six doses of two vaccines in 2006 and half were given placebos. They then got regular tests for the AIDS virus for three years. Of those who got placebos, 74 became infected, while only 51 of those who got the vaccines did.

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