The world's biggest HIV vaccine trial has been slammed by Aids experts.
The Thai trial is due to run for five years
Researchers from some of America's top universities have suggested the trial, involving 16,000 people in Thailand, is a waste of time.
Writing in Science, they said there was no evidence the experimental vaccine will protect people against HIV.
However, the drugs companies involved and the US government, which funds the trial, have rejected the claims saying they believe it is worthwhile.
The $119m Thai study started last year and is expected to run for five years.
The experimental vaccine is made up of a harmless bird virus which doesn't infect humans and a protein found on the surface of the Aids virus called gp120.
The bird virus is used to carry Aids genes into the body. The idea is that their immune system will recognise and attack HIV should they be infected.
However, the Aids experts - from Harvard, UCLA, Cornell University and 15 other respected institutions - say there is no "persuasive data" to suggest that it will work.
They said other trials in Thailand and the US showed that the gp120 element of vaccine does not work.
"The gp120 component has now been proven in phase III trials in the United States and Thailand to be completely incapable of preventing or ameliorating HIV-1 infection," they wrote.
"We seriously question whether it is sensible now to conduct a third trial that, in our opinion, is no more likely to generate a meaningful level of protection against infection or disease."
The experts said continuing with the trial risks damaging public confidence in efforts to find a vaccine for HIV.
The US government agencies funding the trial said they "strongly disagree with the authors' opinions".
They said the results from early trials "justified conducting an efficacy evaluation".
Aventis Pasteur, one of the drug companies involved, said it still backed the trial.
"If I wasn't confident, I wouldn't support it," Dr Jim Tartaglia, its HIV programme director, told BBC News Online.
"We can't be afraid of failing. We have to learn the most from each trial. I know we are going to learn from this trial."