New data on an experimental AIDS vaccine that failed to work shows volunteers who got the shots were far more likely to get infected with HIV through sex or other risky behaviour than those who got dummy injections.

The new details, released Wednesday by drugmaker Merck & Co., don't answer the crucial question of whether the vaccine's failure also spells doom for many similar AIDS vaccines now in testing.

And researchers weren't sure why more of the vaccinated volunteers wound up getting HIV than those who got dummy shots.

Merck, based in Whitehouse Station, N.J., announced Sept. 21 that it was stopping the study because the vaccine didn't work. It was a stunning setback in the push to develop an AIDS vaccine.

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

However, the researchers found that 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 a dummy shot.

Some 3,000 people, mostly homosexual men and female sex workers, volunteered to get the experimental vaccine or a dummy shot. All were warned to protect themselves from AIDS exposure.

At the time the study was halted in September, Merck said 24 of 741 volunteers who got the vaccine in one segment of testing later developed HIV; 21 of 762 participants who got dummy shots also
were infected.

New data released Wednesday showed that to date, 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 became infected.

"In my mind, this doesn't damn anything," Dr. Anthony Fauci, head of the U.S. National Institute of Allergy and Infectious Diseases, said of the vaccine's failure. "It tells you you need to be very careful with every aspect" of vaccine design and testing. The international testing was partly funded by the U.S. National Institutes of Health.

Merck's head of medical affairs for vaccines, Mark Feinberg, said it could be a few years before further data mining and results of other drugmakers' vaccine tests clear up the mystery.