Results from the first successful Aids vaccine trial confirmed yesterday that it is only marginally effective and may weaken over time, prompting a further fierce defence of the value of such costly research.

Scientists who conducted the study, which confounded expectations in the Aids research community when initial findings were released last month, said that it showed the experimental vaccine prevented some HIV infections.

Full details, published in the New England Journal of Medicine, showed that it prevented nearly one-third of infections among 16,000 volunteers in Thailand. Scientists accepted that the results were modest and only showed statistical significance in one type of analysis, but insisted that they held up under careful review.

They added that the work was vital as a guide for directing future research, even if the results were not good enough to justify using the vaccine, known as RV144. Blood samples taken during the trial, sponsored by the US Government and the Thai Ministry of Public Health, were one such clue that might help create a more effective vaccine.

Jerome Kim, a US Army doctor at the Walter Reed Army Institute of Research, who helped lead the trial, described the findings as “a landmark”. “You can put it on a map and begin to figure out where you go from here,” he said, adding that the latest data was a clear validation of last month’s results.

The two-vaccine combination drug lowered the risk of HIV infection by 31.2 per cent among the heterosexual volunteers, who had no special risk of Aids infection. There were 74 infections in the placebo group (those volunteers who did not receive the active drug) and 51 in the vaccine group.

Two further analyses could only suggest that the vaccine is beneficial — rather than providing definitive proof — as they did not reach the threshold of statistical significance.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, the study’s main sponsor, said that this was because so few participants became infected — only 125 people, 10 times less than in previous HIV vaccine trials.

However critics have argued that such tiny numbers mean that the original results may have been a fluke and have questioned whether researchers were over-inflating the significance of their work.

Seth Berkley, president of the International Aids Vaccine Initiative (IAVI), told The Times that he dismissed such criticism, adding that positive trends were shown in all three analyses, offering vital “proof of concept” for vaccine research.

“The most important thing is you know that vaccine production is possible. It will take some time to do further analysis, but the signal [of efficacy] is qualitatively there.”

Scientists said that it was still not known why the vaccine, which comprises a combination of two previously tested vaccine candidates, had worked. The two-vaccine drug was used as part of a “prime-boost” strategy, in which the first drug primes the immune system to attack HIV and the second one strengthens the response. Neither of the vaccine candidates, when tested individually in earlier trials, had shown any ability to protect a person against HIV. As a consequence dozens of scientists had called for the combined-vaccine trial to be disbanded.

The trial of the vaccine, which was tested on HIV-negative men and women aged 18 to 30, began in 2003. The drugs involved were Alvac, made by Sanofi Pasteur, and Aidsvax, which was the subject of one of the biggest vaccine trial failures to date.

Alvac uses canarypox, a bird virus altered by scientists so that it cannot cause human disease, to transport into the body synthetic versions of three HIV genes. Aidsvax contains a genetically engineered version of a protein on the surface of HIV.

The results from the £65 million study, which were presented at a four-day scientific congress on AIDS vaccines in Paris, came as HIV research leaders pleaded with donors to maintain support despite the global financial crisis.

Peter Piot of the Institute for Global Health at Imperial College London and a former head of the United Nations Aids agency UNAIDS, warned that retreat would imperil hard-won progress to provide drugs and craft a vaccine against HIV.

“I’m very concerned that Aids is slipping off the agenda in many countries of the world due to a combination of financial and economic crises,” Dr Piot said. “The money that was spent to save banks, insurance companies and so on is going to have an impact in the social sector and in R&D (research and development).”

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