Another trial of a potential vaccine against AIDS was halted because it was not working, a federal research agency announced Thursday. The trial, known as HVTN 505, started in 2009 with 2,504 gay male volunteers in 19 cities who got three shots over eight weeks. A recent analysis by an outside monitoring board found that more volunteers who got the vaccine became infected during a follow-up period than volunteers who got a placebo. However, the numbers of men infected - 27 vaccine recipients versus 21 placebo recipients - were statistically too close to say that the test vaccine had actually increased the chances of infection, according to the National Institute of Allergy and Infectious Diseases, which sponsored the trial. Those who got the vaccine and became infected anyway also did not have less virus in their blood, the monitoring board found. The vaccine was based on an adenovirus, which causes common colds. In 2007, another trial of an adenovirus-based vaccine was halted because it was not working, and later analysis suggested that for some men - those who had previously caught adenovirus colds - the vaccine made it more likely, rather than less, that they would get infected with H.I.V.