Efficacy of COVID-19 shots in front-line workers tumbled with delta's spread in CDC study

Overall effectiveness of the three COVID-19 vaccines used in the U.S. fell from 91 percent to 66 percent in front-line workers after the delta variant became the dominant form of the virus, according to a study published Aug. 24 by the CDC. But, the researchers noted, it's unclear if the drop is due to the delta variant or if the vaccine's efficacy wanes over time. 

The study included 4,217 front-line workers, including first responders, healthcare workers and others who could not work remotely, 83 percent of whom were vaccinated. Sixty-five percent were vaccinated with Pfizer's shot; 33 percent with Moderna's; and 2 percent with Johnson & Johnson's. Study participants were located in Arizona, Florida, Oregon, Texas, Utah and Minnesota. 

In the months before the delta variant became the dominant form of the virus, the shots were overall 91 percent effective at preventing infection in the study participants. After the variant became dominant, that number fell to 66 percent. Overall, the vaccines reduced infections by 80 percent between Dec. 14 and Aug. 14 compared to unvaccinated workers. Researchers adjusted the results for factors such as occupation, demographic characteristics, frequency of close social interactions and mask use. 

Ashley Fowlkes, an epidemiologist on the COVID-19 response team at the CDC, told The New York Times that "66 percent effectiveness is a really high number. It's not 91 percent, but it is still a two-thirds reduction in the risk of infection among vaccinated participants." 

She added that the drop in effectiveness "should be interpreted with caution" because the period of time in which delta was the dominant form of the virus during the study was short, and the overall number of infections was small. 

Find the full study results here


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