News that one of the most promising and advanced HIV vaccine trials has been halted has dealt a serious blow to global AIDS prevention efforts.

The vaccine's developer, Merck, announced that it was ending enrollment and vaccination of volunteers in the US National Institutes of Health-funded study following a lack of evidence the vaccine had lowered the risk of HIV infection or reduced the severity of infection among volunteers who became infected during the trial.

The data was drawn from phase II clinical trials conducted in North and South America, the Carribean and Australia, which began in December 2004. The volunteers were mostly homosexual men and sex workers considered at high risk of contracting HIV. Among 741 people who received at least one dose of the vaccine, 24 cases of HIV infection were found after 13 months compared to 21 infections among 762 volunteers who received a placebo. The vaccine also failed to reduce the amount of virus in the bloodstream in those who became infected.

A second phase II study of the vaccine, which began enrolling volunteers in South Africa early this year, has also been discontinued. The South African trial, called "Phambili", had already recruited 700 volunteers who had begun to receive a series of three injections.

In local media reports, chief investigator of the South African trials, Dr Glenda Gray of the University of the Witwatersrand Perinatal HIV Research Unit (PHRU) in Soweto, emphasised that none of the volunteers had been put at any risk as the vaccine's safety had already been demonstrated in earlier studies. But Manto Tshabalala-Msimang, South Africa's health minister, was quoted in a local newspaper as expressing concern that such trials were being done in "haste" and could be putting lives at risk.

The serum was part of a new class of HIV vaccines that scientists have pinned their hopes on since earlier attempts to develop a vaccine using more traditional approaches failed to yield results. It used a weakened version of the common cold virus to deliver three synthetically produced genes from the HI virus with the goal of stimulating the body's immune response when exposed to the real disease. Although preliminary results found the vaccine did trigger an immune response, a data safety monitoring board found it did not prevent HIV infection. The implications of the findings for other vaccines that rely on similar approaches are not yet known.

"This is a huge disappointment for all of us who have been involved in the search for an HIV vaccine," said Gray in a statement issued by Merck. "The scientific community must continue the race to find a vaccine to help secure an HIV free generation for the future."