Bangkok, Sep 24 (DPA) Thai health authorities Thursday announced the first 'effective' trial of an HIV/AIDS vaccine on some 16,000 volunteers, although the efficacy rate was not sufficient for use.

'It is found that the vaccine has 31.2-per-cent efficacy in reducing the risk of HIV infection,' Thai Health Minister Withaya Kaewparadai told a press conference.

'The outcome represents a breakthrough in HIV vaccine development because for the first time ever there is evidence that HIV vaccine has preventative efficacy,' he added.

While describing the trial outcome as a positive step forward in the so-far unsuccessful search for an HIV/AIDS virus, Withaya acknowledged that the efficacy rate was not high enough to put the vaccine into use.

The results were part of the HIV Vaccine Trial Phase III on 16,402 Thai volunteers in Chonburi and Rayong provinces. Half of the volunteers were given the RV 144 vaccine in 2006, and the other half received placebos.

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.