A human test of the U.S. government's experimental AIDS vaccine, similar to a failed Merck & Co. product, was canceled after a top scientist determined it was unlikely to give useful results.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Md., said yesterday he was unwilling to contribute the resources needed for the trial. The test, PAVE-100, was set to include about 3,000 people, and was originally planned to enroll as many as 8,000.

Fauci went against the recommendation of an advisory panel that voted May 30 in favor of conducting the test in some form. He said he would consider a smaller trial. The government's vaccine is intended to stimulate immune cells to reduce or eliminate levels of HIV in the blood.

"Given the fact that there are not a lot of leads on an AIDS vaccine, I'm not willing to entirely shelve the concept," Fauci said in a telephone interview. "But it would have to be a leaner, meaner, less-expensive trial, with less people, that's focused on the question of whether the vaccine can lower viral load."

Merck, whose vaccine-making operations are in West Point, Montgomery County, developed a vaccine called Ad5, also intended to lower the amount of virus in the bloodstream. An international test of the vaccine in about 3,000 people was halted in September when 49 HIV infections occurred among those who received it, while just 33 who got placebo vaccinations contracted HIV. That suggested the vaccine may have inadvertently increased HIV risk among people who were exposed to blood or semen containing the virus.

Both the Merck vaccine and the government's experimental shot, called VRC for the Vaccine Research Center where it was developed, contain a cold virus called adenovirus-5. Because of that similarity to the Merck vaccine, the VRC shot probably wouldn't have gained market clearance, scientists said.

At the May 30 advisory meeting, scientists argued that PAVE-100 trial should still proceed because it might yield information about how the human immune system reacts to components of the vaccine that activate immune T-cells. A study of a vaccine that would never reach the market would be difficult to explain to people being enrolled, said Mitchell Warren, executive director of the AIDS Vaccine Advocacy Coalition in New York.

"We said that we wanted to see the smallest, most-efficient trial to answer the question," he said yesterday from South Africa. "PAVE-100 wasn't seen as small or efficient enough."