Of those who got placebos, 74 became infected, while only 51 of those who got the vaccines did.

The RV 144 combined two vaccines that had previously proved ineffective in immunising people against HIV/Aids - Alvac-HIV and  Aidsvax.

The trial, deemed the world's largest HIV vaccine study, was sponsored by the US Army Surgeon General with funding from the National Institute of Health and the US Army Medical Research.

Thailand has an estimated 600,000 people who are living with HIV/Aids. About 400,000 Thais have died of Aids-related diseases since the virus was first detected in the country in the mid-1980s.

Thailand has been deemed a success story for its anti-HIV/Aids programmes, which have included public awareness campaigns, passing out free condoms at brothels and making anti-retro viral drugs available at subsidized prices to many of those infected with the virus.

"Even if an efficacious HIV vaccine may eventually materialize in the future, everybody must protect themselves and avoid risky behavior," Doctor Somchai Chakkrabhand, director-general of Thailand's Department of Disease Control, cautioned.

The two main United Nations agencies dealing with HIV/Aids said they were "optimistic" about the results.

The results, the World Health Organization and the UNAids agency said, were "of great importance."

The 31.2-per-cent vaccine efficacy in preventing HIV infections offered by the dose was deemed by the health agencies to be "modestly protective."

"Much more work, though, has to be done by the principal investigators and a large group of international collaborators to analyse the trial data, understand the protective mechanism, determine the duration of protection, and map next steps," the agencies said.

Also, the UN urged that once an HIV vaccine does become available, "it will need to be universally accessible by all persons at risk."