One of the leading experimental AIDS vaccines has failed to prevent test subjects from becoming infected with HIV, subsequently prompting its sponsor to abruptly shut down the trial.

Merck & Co., testing their AIDS vaccine in a network funded by the National Institutes of Health, or NIH, had spent a decade developing the vaccine.

Executives 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 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."

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 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.

The results are particularly disappointing because it is widely believed that only a vaccine can end the epidemic. Last year, more than four million people world-wide contracted HIV and nearly three million died, according to United Nations estimates.

Almost 40 million people are currently living with HIV.

But Merck's vaccine is one of many in or heading into clinical trials, and different types of vaccines are known to stimulate different kinds of immunity. One experimental immunization now in human trials developed by the HIV Vaccine Research Center of the NIH had shown more-promising results in monkey trials than did the Merck vaccine.

"It isn't the end of the line," Mitchell Warren, executive director of the AIDS Vaccine Advocacy Coalition, a New York group advocating prevention, told the Wall Street Journal. Merck's data "aren't the answers we wanted, but they will help improve our other vaccine candidates."

The big fear among researchers, according to the Journal, is that the whole theory underlying the Merck vaccine might be flawed, and that may doom an entire class of experimental vaccines.

Most vaccines, such as those used against smallpox or polio, stimulate the body to produce antibodies that ward off infection. But stimulating antibodies that neutralize a broad range of HIV strains has proved elusive, so researchers focused on the other arm of the immune system: the so-called killer T-cells, which attack and kill cells that HIV has already infected. Such vaccines have been considered less likely to prevent someone from getting infected. It was hoped, however, they would enable an infected person to suppress the virus and so delay, perhaps indefinitely, the onset of disease.

The Merck vaccine did stimulate the immune system's T-cells but not in a way that helped infected test subjects control the virus. Now, researchers will try to figure out why.

source: Wall Street Journal