(NBC NEWS) -- Bad news in the fight against the AIDS virus: The government is halting a large U.S. study of a possible HIV vaccine because the experimental shots aren't preventing infection.
The study had enrolled about 2,500 people, mostly gay men, in 19 cities. Half received an experimental vaccine developed by the National Institutes of Health, and half received dummy shots.
A safety review this week found that slightly more volunteers who had received the vaccine later became infected with HIV. It's not clear why.
The NIH said Thursday that it is stopping vaccinations, but will continue to study the volunteers' health.
Multiple attempts at creating an AIDS vaccine have failed over the years. But researchers continue to try, pointing to modest success in a 2009 study in Thailand.