On Jan. 26, 2022, the NIH announced that adults who had previously received a full regimen of any of three COVID-19 vaccines granted Emergency Use Authorization or approved by the U.S. Food and Drug Administration, an additional booster dose of any of these vaccines was safe and prompted an immune response, according to preliminary clinical trial results reported in The New England Journal of Medicine.

The findings served as the basis for recommendations by the FDA and the Centers for Disease Control and Prevention in late fall 2021 to permit mix-and-match COVID-19 booster vaccinations in the United States.