Associated Press
Saturday, September 22, 2007; A08

TRENTON, N.J., Sept. 21 -- 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 the enrollment and vaccination of volunteers in the study, which was partly funded by the National Institutes of Health.

It was a high-profile failure in the daunting quest to develop a vaccine against AIDS. Merck's vaccine was the furthest along and was closely watched by experts in the field.

Officials at the company 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.

Michael Zwick, an HIV researcher at Scripps Research Institute, said it is too soon to know whether 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 participants were all free of HIV at the start and at high risk for contracting the virus: Most were homosexual men or female sex-workers. Merck said all were repeatedly counseled about how to reduce their risk of HIV infections, including condom use.

In a statement, the NIH said a data safety monitoring board, reviewing interim results, found that the vaccine did not prevent infection or limit the 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.