The trial team in Bangkok, Thailand's capital announced on 24 September that rates of HIV infection were 31 percent lower in trial participants who got the vaccine than in those who received a placebo.

"These new findings represent an important step forward in HIV vaccine research," said Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases (NIAID), the main funder of the trial.

The study, known as RV144, began enrolling 16,000 HIV-negative men and women between the ages of 18 and 30 in October 2003. Half the volunteers received a placebo; the other half were given shots containing two different vaccines. The first, called ALVAC-HIV, used a disabled form of a bird virus known as canary pox to deliver synthetic versions of three HIV genes into the body. The second, called AIDSVAX, was composed of a genetically engineered version of an HIV protein.

The synthetic HIV components in both vaccines were based on subtypes B and E of the virus, which are most common in Thailand, the US and Europe. Scientists do not yet know whether the vaccine would be effective against other strains, such as subtype C, which is most prevalent in sub-Saharan Africa.

The trial was designed to evaluate whether the combined vaccines lowered HIV infection risk, and whether they had any impact on viral load [the amount of HIV circulating in the bloodstream] in the volunteers who became infected.

Of 8,197 people given the vaccine regimen, 51 became infected, compared to 74 of the 8,198 volunteers who received the placebo. This result is considered "statistically significant", meaning that the difference is unlikely to be a coincidence. The vaccine did not have any effect on viral load.

"Today's result is not the beginning of the end of the epidemic, it's the end of the beginning of finding an AIDS vaccine. It's a thrilling moment," Mitchell Warren, executive director of the AIDS Vaccine Advocacy Coalition (AVAC), told IRIN/PlusNews on the phone from New York. However, he emphasized that additional studies and analysis were needed to confirm and understand the findings.

The vaccine's modest effectiveness means it is unlikely to be licensed or produced in large quantities in Thailand, where the rate of HIV infection is relatively low. However, Prof Gavin Churchyard, CEO of the Aurum Institute, a non-profit medical research organization based in South Africa, said even an AIDS vaccine that was only 30 percent effective could have an impact in southern Africa, where HIV infection rates are much higher, "but we would need to know if it would work in this population".

Churchyard said the results had come as a surprise to many in the vaccine field. "We weren't actually expecting a positive result," he commented. Previous efficacy trials of AIDSVAX, the second vaccine in the regimen, had found no benefit and the decision to go ahead with the large-scale trial in Thailand had generated controversy.

Warren noted that vaccine science had evolved considerably since the trial was launched in 2003. "There are new ideas and approaches that no one imagined six years ago. Anytime you start a trial, it's like buying a new computer - it's outdated before you even get it out of the box." He added that whether or not the approach used in the trial was determined to be the most effective, the findings would still influence future strategies.

Good news at last

The positive results from the Thai trial are expected to give a crucial boost to a field in desperate need of good news after a series of setbacks in recent years. A four-continent trial of a vaccine developed by pharmaceutical company Merck was halted in 2007 after preliminary results suggested that it not only did not provide protection against HIV, but might actually increase the risk of infection.

Dr Glenda Gray of the Perinatal HIV Research Unit (PHRU) at the University of the Witwatersrand in Johannesburg, chief investigator in the South African arm of the Merck vaccine trial, told IRIN/PlusNews the outcome in Thailand was "a huge step forward - it opens up the field again and gives us an indication that this [a vaccine] is possible."

The results are also significant for the future of two HIV vaccines that began small-scale human trials in South Africa in July. One of the vaccines uses components from the family of pox viruses similar to those used in one of the Thai vaccines. "It means, hopefully, there'll be more interest in our vaccine," said Gray, the lead investigator of the trials being conducted by the South African AIDS Vaccine Initiative (SAAVI) and NIAID.

"We are planning a larger trial next year and having these results makes it much easier for us to convince funders to go ahead with the next phase," Gray said.

More information on the Thai trial results will be presented at an AIDS vaccine meeting in Paris in October.

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