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.

«Not the beginning of the end of the epidemic»

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.

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.