Plans for a large human trial of a promising government-developed H.I.V. vaccine in the United States were canceled Thursday because a top federal official said scientists realized that they did not know enough about how H.I.V. vaccines and the immune system interact.

The decision is a major setback in an effort to develop an H.I.V. vaccine that began 24 years ago when government health officials promised a marketed vaccine by 1987. Health officials have long contended that such a vaccine would be their best weapon to control the AIDS pandemic.

A number of other H.I.V. vaccines are in various stages of testing around the world. But there had been high hopes for the government’s trial because the potential vaccine was among a new class that sought to stimulate the immune system in a different way.

The official who canceled the government trial, Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said it was becoming clearer that more fundamental research and animal testing would be needed before an H.I.V. vaccine was ever marketed.