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.
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 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.
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 three 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 is 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, cosponsored 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 US.
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.
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