COVID-19 vaccines developed by Pfizer Inc. with BioNTech SE and Moderna Inc. reduced risk of infection by 80% two weeks or more after the first of two shots, according to data from a real-world U.S. study released on Monday.

The risk of infection fell 90% by two weeks after the second shot, the study of nearly 4,000 U.S. health care personnel and first responders found.

The results validate earlier studies that had indicated the vaccines begin to work soon after a first dose, and confirm that they also prevent asymptomatic infections.