In another major setback for efforts to develop an HIV vaccine, federal researchers have shut down a key clinical trial after an independent panel of safety experts determined that volunteers who got an experimental vaccine appeared to be slightly more likely to contract the human immunodeficiency virus than those who got a placebo.

Investigators involved in recruiting volunteers and running the trial at 21 sites across the country were ordered Tuesday morning to stop immunizing volunteers with the genetically engineered HVTN 505 vaccine and to inform the nearly 2,500 people who participated in the study whether they got the vaccine or the placebo. All of the volunteers were men or transgender people who have sex with men.