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 subsidised prices to many of those infected with the virus.

'Even if an efficacious HIV vaccine may eventually materialise in the future, everybody must protect themselves and avoid risky behaviour,' 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-percent 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'.