In a major setback, one of the leading experimental AIDS vaccines has failed to prevent test subjects from becoming infected with HIV.

To make the situation worse it could not even offer any indication it might delay the onset of full-blown AIDS, which had been a key hope.

The collapse of the trial leaves Merck & Co., which had spent a decade developing the vaccine, with no remaining prospects in the global hunt for an AIDS immunization. The vaccine was tested in a network funded by the National Institutes of Health.

"We've been kicked in the teeth," said Bruce Walker, a veteran AIDS researcher at Harvard University who wasn't involved in the study. Lawrence Corey, a leader of the HIV Vaccine Trials Network, said he was "mourning."

The results are particularly disappointing because it is widely agreed that only a vaccine could end the epidemic. Last year, more than four million people worldwide contracted HIV, the virus that causes AIDS, and nearly three million died, according to United Nations estimates. Almost 40 million people are currently living with HIV.

The trial was stopped early by independent overseers known as the Data & Safety Monitoring Board. Comparing two groups -those who received the vaccine and those who received a placebo- the overseers determined there was virtually no statistical difference in infection rates between them, indicating the vaccine was not working. In addition, the amount of HIV in the blood of those who did get infected, a predictor of how fast a person will get full-blown AIDS, was virtually the same in each group.

But researchers cautioned against overreacting. "It isn't the end of the line," said Mitchell Warren, executive director of a NY-based AIDS Vaccine research group, "Merck's data aren't the answers we wanted, but they will help improve our other vaccine candidates."

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