The mRNA molecule eventually breaks down in the body. The Cleveland Clinic reports that the risks of mRNA vaccines include pain or swelling at the injection site, fever, fatigue, headaches, muscle aches and allergic reactions.

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.