the vaccine to cause HIV infection because it carried so little of the virus.

The study started 18 months ago after 20 years of research and development to get the vaccine to human trials.

Its chief researcher in Australia, Dr Tony Kelleher from the University of New South Wales, told ABC reporter Karen Barlow in May last year there was no chance for the vaccine to cause HIV infection because it carried so little of the virus.

Researchers say that is still the case but somehow things went horribly wrong with the phase-two trial.

Of the people who had the HIV vaccine, 49 became infected with the virus, whereas only 33 in the placebo group became infected.

The vice-president of Merck's research team, Keith Gottesdiener, says the trial was abruptly stopped two months ago because V520 was not preventing infection.

"We are analysing the data to try to determine if the results are due to immune responses induced by the vaccine, differences in study populations, or some other biological phenomenon we don't yet understand, or simply due to chance," he said.

"It will take some time before we understand why the vaccine did not work and why there was a trend toward more cases of infection in volunteers who received the vaccine."

Doctor Larry Corey from the US Vaccine and Infectious Disease Institute says its too soon to establish whether the virus caused the increased infection rates.

"One of the possibilities is that the increase in the number of infections could be related to the vaccine, there are many other possibilities as well," he said.

"My own opinion is that it's way too early to really answer and the data is way too complex and there's no simple answer to that question."

Cold virus

The trial vaccine used a disabled form of the common-cold virus to carry three synthetically produced HIV genes into the body.

It appears people who had higher levels of immune protection against the virus before getting the vaccine were at highest risk of acquiring HIV.

The vaccine did not contain live HIV. It had been well tolerated in smaller clinical trials and had produced immune responses.

The trial was also conducted in North America, South America, the Caribbean and South Africa.