The targeting of the vaccine technology once considered revolutionary follows intense criticism by conservatives and anti-vaccine activists.

Kennedy said Tuesday that the terminated contracts were worth nearly $500 million. Among them is an award to the University of Texas Medical Branch and the biotechnology company Moderna to develop an mRNA-based vaccine for H5N1, the strain of bird flu that has infected dozens of people in the United States.

The Trump administration cancelled about $500 million for research into mRNA vaccines. The move slows progress in using the technology to prevent a future pandemic or treat disease, experts say.