Merckhas halted testing of its vaccine to prevent infection with HIV -- long considered among the most promising vaccines in development -- after a monitoring board found it was ineffective, the company said.
The failed tests represent a major setback in the global effort to stem infections with the virus that causes AIDS.
Shares of Merck were little affected by the news.
Merck had expressed great optimism for the vaccine, which it has been testing for a decade. The independent Data Safety Monitoring Board, after reviewing interim results of the study of the vaccine, recommended discontinuing vaccinations of volunteers as the trial was headed for failure, Merck said.
The company, which was developing the product in partnership with the federally funded HIV Vaccine Trials Network, said two other early-stage trials of the vaccine had also been discontinued.
Study investigators have been instructed to discontinue vaccinating volunteers and to monitor them, Merck said.
The study was aiming to determine whether the vaccine prevented HIV infection and whether it reduced the amount of virus in those who developed infection.
Although a number of medicines have been introduced in recent years that control the virus and keep symptoms of the otherwise fatal disease at bay, doctors have said a preventive vaccine is by far the best way to control continued spread of the disease.
Millions of people have succumbed to the disease since it started becoming prevalent in the early 1980s, with the worst outbreaks in Africa. The virus is now believed to be spreading quickly in India, China and other parts of Asia, fueled by unprotected sexual contact, prostitution and shared needles among drug users.
As many as 1 million Americans are believed infected with the virus. Many have insurance that covers the costly drug cocktails needed to prevent uncontrolled growth of the virus.
But the drugs, which can cause serious side effects, must be taken for life because they cannot totally eradicate the quickly mutating virus.
Merck's discontinued international trial, called STEP, involved 3,000 HIV-negative volunteers from diverse backgrounds, between the ages of 18 and 45, at high risk of HIV infection.
An interim efficacy analysis, conducted in about 1,500 volunteers expected to have the best response to the vaccine, showed the drug was ineffective.
Although the vaccine has been a major hope among the medical community, it was not deemed to have major commercial potential.