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.
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 study by the 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.
The findings from the real-world use of these messenger RNA (mRNA) vaccines also confirm the efficacy demonstrated in the large controlled clinical trials conducted before they received emergency use authorizations from the U.S. Food and Drug Administration.
The study looked at the effectiveness of the mRNA vaccines among 3,950 participants in six states over a 13-week period from Dec. 14, 2020, to March 13, 2021. About 74% had at least one shot, and tests were conducted weekly to catch any infections without symptoms.
CDC Director Rochelle Walensky said in a statement, “The authorized mRNA COVID-19 vaccines provided early, substantial real-world protection against infection for our nation’s healthcare personnel, first responders, and other frontline essential workers.”
CDC said the study results provide reassurance that people start to develop protection from the vaccine two weeks after their first dose, although the agency reiterated that the greatest protection was seen among those who had received both recommended doses of the vaccines.