In a disappointing setback, a promising experimental AIDS vaccine failed to work in a large international test, leading the developer to halt the study.

Merck & Co. said Friday that it is ending enrollment and vaccination of volunteers in the study, which was partly funded by the National Institutes of Health.

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It was a high-profile failure in the daunting quest to develop a vaccine to prevent AIDS. Merck's vaccine was the farthest along, considered the most promising and was closely watched by experts in the field.

Officials at the company, based in Whitehouse Station, N.J., said 24 of 741 volunteers who got the vaccine in one segment of the experiment later became infected with HIV, the virus that causes AIDS. In a comparison group of volunteers who got dummy shots, 21 of 762 participants also became infected.

"It's very disappointing news," said Keith Gottesdiener, head of Merck's clinical infectious disease and vaccine research group. "A major effort to develop a vaccine for HIV really did not deliver on the promise."

Michael Zwick, an HIV researcher at Scripps Research Institute, said it's too soon to know if other vaccines using the same strategy would also fail.

"It's par for the course in the HIV field," he said of the Merck result.

The volunteers in the experiment were all free of HIV at the start. But they were at high risk for getting the virus: Most were homosexual men or female sex workers. They were all repeatedly counseled about how to reduce their risk of HIV infections, including use of condoms, according to Merck.

In a statement, the NIH said a data safety monitoring board, reviewing interim results, found the vaccine did not prevent HIV infection. Nor did it limit severity of the disease "in those who become infected with HIV as a result of their own behaviors that exposed them to the virus" — another goal of the study.

Merck's was the first major test of a new strategy to prevent HIV infection. The first wave of attempts to develop a vaccine tried to stimulate antibodies against the virus, but that hasn't worked so far.

The new effort — an approach that Gottesdiener said is being tried in most other current research — is aimed at making the body produce more of a crucial immune cell called killer T cells. The goal is to simultaneously "train" those cells, like an army, to quickly recognize and destroy the AIDS virus when it enters cells in the bloodstream.

Dr. Anthony Fauci, head of NIH's National Institute of Allergy and Infectious Diseases, said other vaccines are in line for testing that use a similar strategy but with important differences. One developed by the NIH — containing the same three genes as the Merck vaccine, plus 3 versions of another gene — is set in January to enter the same large, "proof of concept" testing stage as the Merck vaccine, Fauci said.

Zwick said some researchers still are working on vaccines to neutralize the AIDS virus. He thinks ultimately what's needed is one that combines that approach with a way to stimulate and train killer T cells.

Merck and the HIV Vaccine Trials Network, an international collaboration of researchers and institutions funded by the NIH, co-sponsored the study. The experiment, called STEP, began in December 2004 and had enrolled 3,000 volunteers in Australia, Brazil, Canada, the Dominican Republic, Haiti, Jamaica, Peru, Puerto Rico and the United States.

The results announced Friday involved volunteers who researchers thought would benefit most because they had never been exposed to the particular cold virus used in the vaccine.

Wall Street, on a generally upbeat day, showed little reaction to the news, with Merck shares rising 44 cents to $51.82.

Analyst Steve Brozak of WBB Securities said the failure won't hurt Merck's bottom line in the short term. But he said a vaccine is the only financially feasible way to fight the AIDS epidemic in poor countries and that the company that comes up with the first successful shot would have "a license to print money."

"You're talking about a Carl Sagan kind of number — billions and billions" of dollars, he said.

The Merck vaccine, known only as V520, also was being tested in a similar study in South Africa and in two smaller studies, which also were halted.

Mark Feinberg, head of medical affairs in Merck's vaccine division, said it has no other candidates in development. The company still must do a detailed review of the data but he said he thinks this approach is dead.

Other scientists disagreed.

"You can't say that the whole field of T cell vaccines has been dealt a death blow" by the Merck results, Fauci said. But if further examination of the Merck data and research on similar vaccines show that's the case, he said, "then we're in some serious trouble because the candidates representing different vaccine concepts are years away from development."

Dr. Wayne Koff, head research and development at the International AIDS Vaccine Initiative, said the results give important data on what is not going to work and researchers will build on that as other potential vaccines are tested. He said roughly 30 are in the pipeline.

The nonprofit AIDS Vaccine Advocacy Coalition said in a statement that "while this is a disappointment, it is in no way the end of the search for an AIDS vaccine."