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. healthcare 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.
Some countries dealing with limited vaccine supplies have pushed back schedules for second doses with the hope of getting some protection to more people. U.S. public health officials, however, continue to recommend two doses be given on the schedule authorized by regulators based on clinical trials.
The study by the U.S. Centers for Disease Control and Prevention (CDC) evaluated the vaccines’ ability to protect against infection, including infections that did not cause symptoms. Previous clinical trials by the companies evaluated their vaccine’s efficacy in preventing illness from COVID-19, but those studies would have missed asymptomatic infections.