The global fight against AIDS has been dealt a blow with the news that Merck & Co has halted clinical trials on its HIV vaccine V520, generally considered one of the most promising in development.

Merck officials announced that the adenoviral vaccine conspicuously failed to protect volunteers in high-risk groups, including sex workers and gay men. The company had expressed high hopes for the jab, which it spent 10 years developing and experts in academia and at organisations such as the International AIDS Vaccine Initiative, had also been optimistic.

Merck's international trial, called Step, began in 2004 and involved 3,000 HIV-negative volunteers from diverse backgrounds, between the ages of 18 and 45 in North and South America, the Caribbean and Australia.

24 of 741 volunteers who got the vaccine became infected with HIV, while out of a group of 762 volunteers who were given a dummy version of the jab, 21 became infected.

In addition, the vaccine did not reduce the amount of virus in the bloodstream of those who became infected, indicating that it would not slow the onset of AIDS, as Merck had hoped. An independent monitoring panel recommended discontinuing the vaccination of volunteers, saying the trial was headed for failure.

"This is a huge disappointment for all of us who have been involved in the search for an HIV vaccine," said Dr Glenda Gray, principal investigator of an intended follow-up trial in South Africa, which has also now been cancelled. "HIV is ravaging our communities, and all the scientists, participants and communities involved in HIV vaccine studies have been affected by this epidemic. The scientific community must continue the race to find a vaccine to help secure an HIV-free generation for the future."

Peter Kim, the president of Merck Research Laboratories, said: "We share in the disappointment of the research and HIV communities today. Sadly, developing an effective AIDS vaccine remains one of the most challenging tasks facing modern medicine. We are committed to studying the data closely and sharing it with the scientific community to inform the on-going search for an effective HIV vaccine." Larry Corey, principal investigator of the HIV Vaccine Trials Network, which sponsored the studies, said: "While we are very disappointed that this vaccine candidate did not demonstrate protection, the data from this trial will provide critical insights into this disease and future vaccine development."

The vaccine used a cell-mediated immune response approach; it was hypothesised that the HIV genes in the vaccine would stimulate the body to generate a potent HIV-specific T-cell immune response against infected cells. Some scientists have argued, however, that to prevent infection, a vaccine will also have to prompt an antibody response against the virus.

But the urgent need for a vaccine, even a partially effective one, is not in doubt. In the past 20 years, AIDS has claimed 40 million lives, says UNAIDS, the joint United Nations initiative on HIV and AIDS. Last year, 2.9 million people died from AIDS, 2.1 million of them in sub-Saharan Africa, and over 20 million people are currently thought to be infected with the virus By Michael Day.