TRENTON - 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 on Friday that it is ending enrollment and vaccination of volunteers in the study, which was partly funded by the National Institutes of Health (NIH).

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, New Jersey, 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 is 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 behaviours that exposed them to the virus' - another goal of the study. -- AP