A major HIV vaccine trial in South Africa was brought to a halt Friday after interim results from a sister trial in the United States and Australia suggested it failed to prevent against HIV infection.

The news came as a major blow to AIDS researchers as the vaccine had been billed the world's most promising AIDS vaccine candidate.

The South African trial was at the most advanced clinical stage of any HIV vaccine trial on the African continent.

"It's been a very bad day for us. We all feel terribly despondent at the news," Dr Glenda Gray, chief investigator in the South African trial of the Merck MRKAd5 vaccine based at Chris Baragwanath Hospital in Soweto said in an interview with Deutsche Presse-Agentur dpa.

Seven hundred people had enrolled in the trial code-named Phambili that began at five sites across South Africa in February 2007.

The trial sought to test the efficacy of the vaccine in fighting the C subtype of the HIV virus found in Africa after preliminary results from trials in other parts of the world had suggested it had some success in fighting the B subtype strain.

But the trial was paused Friday after interim results from a trial carried out in regions with the B subtype, including the US and Australia, showed it probably did not protect against HIV infection.

The organizers of the South African trial would take a month to examine the data from the sister trial before deciding whether to continue with the Phambili trial in a modified form or suspend it, Gray said.

The researcher stressed that the safety of the volunteers in the trial was never in jeopardy as the vaccine's safety had been borne out in earlier testing.

"We do know it's safe and that it won't cause HIV but we don't know if it protects against HIV infection," she said.

Preliminary results from the Phamibili trial had shown a good immune response but it now looked as if a good immune response might not necessarily lead to HIV protection, Gray explained.

South African researchers would examine whether factors such as, gender and means of contracting HIV may have influenced the results of the sister trial. Over 90 per cent of the volunteers in that trial were male. The South African trial is more evenly split between men and women.

Two other HIV vaccine trials are underway in South Africa, one aimed at prevent the transmission of HIV, another aimed at treating its symptoms.

South Africa has the largest number of HIV-positive people worldwide. Some 5.4 million South Africans, or one in nine, are infected. dpa cb ds