The effectiveness of the Pfizer and Moderna vaccines against Covid-19 has fallen from 91 percent to 66 percent since the delta variant became dominant in the United States. This is according to data released by US health authorities on Tuesday.
The data comes from a survey of thousands of health professionals and hospital workers in six states examining the performance of vaccines in real-world conditions. Participants are tested weekly for both asymptomatic and asymptomatic infections.
Between December 2020 and April 2021, the vaccine’s effectiveness in preventing infection was 91 percent, according to data from the Center for Disease Control and Prevention (CDC), the leading federal public health agency. But in the weeks when the delta variant became responsible for more than 50 percent of infections, the efficacy dropped to 66 percent.
However, the study authors caution that this decrease could be due not only to the delta variable, but also to a decrease in efficiency over time. “While these interim data indicate a moderate decrease in the efficacy of Covid-19 vaccines in preventing infection, the fact that the two-thirds reduction in infection continues to underscore the continued importance and usefulness of vaccination,” the authors wrote.
The data released Tuesday does not differentiate the effectiveness of Moderna’s vaccine and Pfizer’s serum.
This decrease in efficacy against the delta variant has been indicated by various studies, although the exact number varies from study to study.