July 17 (Bloomberg) -- 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, Maryland, said today 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, the AIDS virus, 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, based in Whitehouse Station, New Jersey, developed a vaccine called Ad5, which was 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 caught HIV. That suggested the vaccine may have inadvertently increased HIV risk among people who were exposed to blood or semen containing the virus.

Cold 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 have 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 today in a telephone interview from South Africa. ``PAVE-100 wasn't seen as small or efficient enough.''

About 33 million people in the world have the AIDS virus, and 2.5 million new infections occur each year. The virus can quickly develop resistance to treatments, which are expensive to develop, and researchers are looking for preventive measures.

`Important Issue'

The failure of the Merck vaccine shows that more studies of the interaction between the human immune system and HIV are needed to design effective prevention, Fauci said. He called today for more studies of immune proteins known as neutralizing antibodies that are activated by most effective vaccines.

``We still have to answer some of the fundamental questions of how we might induce neutralizing antibodies,'' he said. ``That's still the big, important issue.''

Fauci said in May that his agency would give $15.6 million to 10 laboratories across the country studying antibodies that can neutralize HIV by preventing the virus from entering cells.

``This in no way signals the end of the search for an AIDS vaccine,'' said Warren of the AIDS Vaccine Advocacy Coalition. ``This has to do with the investigation of this candidate.''

To contact the reporter on this story: John Lauerman in Boston at jlauerman@bloomberg.net.

Last Updated: July 17, 2008 11:41 EDT