The effectiveness against infection of the Pfizer and Moderna COVID-19 vaccines dropped from 91 percent before the Delta variant became dominant to 66 percent afterwards, according to a large study of U.S. health workers published Tuesday.
The Centers for Disease Control and Prevention (CDC) has been examining the real-world performance of the two vaccines since they were first authorized among healthcare personnel, first responders and other frontline workers.
Thousands of workers across six states were tested weekly and upon onset of COVID-19 symptoms, allowing researchers to estimate efficacy against symptomatic and asymptomatic infection.
By looking at the rate of infections among vaccinated and unvaccinated people and the amount of time they were tracked, vaccine effectiveness was estimated at 91 percent in the initial study period of December 14, 2020 to April 10, 2021.
But during weeks in the run-up to August 14, when the ultra-contagious Delta variant became dominant, effectiveness fell to 66 percent.