In a puzzling and potentially troubling development, an AIDS vaccine tested in a closely watched trial might have increased the risk among vaccine recipients of becoming infected with H.I.V., researchers reported yesterday at a scientific meeting in Seattle.

But the researchers said not enough data existed to determine the meaning of the findings about the vaccine, which is made by Merck.

The increased risk was principally among a group of people who had pre-existing levels of immunity to a common cold virus known as adenovirus type 5, which was modified to become a critical part of the vaccine. Researchers emphasized that the vaccine itself could not cause AIDS, but one theory is that the cold virus may have activated the immune system in some way to make certain recipients more susceptible to becoming H.I.V.-infected when they were exposed to the AIDS virus.

But participants at the meeting also emphasized that the findings could be a statistical fluke and that nonbiological factors might have accounted for the difference. Examples of such factors are rates of circumcision and sexual practices among trial participants.