An experimental HIV/AIDS vaccine that was withdrawn from trials in September may have increased HIV infection risk among test volunteers, new reports say. The vaccine, which was being developed by Merck, the National Institute for Allergy and Infectious Diseases and the HIV Vaccine Trials Network, failed because it contained a cold germ that appears to weaken immunity.

The vaccine is made from a common cold virus with three synthetic HIV genes which stimulate the immune system to kill any HIV-infected cells encountered in the future.

The volunteers with pre-existing immunity to this particular cold virus were much more likely to get infected with HIV if they got the AIDS vaccine than if they got the dummy shot, researchers said.

Dr. Keith Gottesdiener, vice president of clinical research at Merck Research Laboratories told the AP, "One of the possibilities is that the increase in the number of infections was related to the vaccine."

Merck, based in Whitehouse Station, N.J., announced on Sept. 21 that it was stopping the study because the vaccine didn't work, thus providing a major blow to the work in the field to develop an AIDS vaccine.

Among the 3,000 people who participated in the trials, a large subset who was vaccinated, developed HIV at higher rate than those who received a placebo.

According to latest data, 49 of 914 vaccinated men became infected with HIV, compared with 33 of the 922 men who got dummy shots. Only one woman and a small number of heterosexual men were infected.

Vaccinated people who got the shots were also far more likely to get infected with the virus through sex or other risky behavior.