The effectiveness of Covid-19 vaccines among front-line workers dropped to 66% after the delta variant became dominant, compared to 91% before it emerged, according to a report from the Centers for Control and the US Disease Prevention.
Vaccines remain protective, the CDC said. The finding should be interpreted with caution, as the vaccine’s effectiveness could decline over time, and efficacy estimates were imprecise.
“Although these interim findings suggest a moderate reduction in the effectiveness of Covid-19 vaccines in preventing infections, the sustained two-thirds reduction in the risk of infection underscores the importance and continued benefits of Covid-19 vaccination,” wrote the Researchers in Morbidity and Weekly Mortality Report.
The findings echo previous evidence from Israel and the United Kingdom suggesting that Covid vaccines lost some potency to prevent infections over time as the delta variant spread. These and other research results will come under scrutiny next week. CDC advisers weigh the Biden administration’s plan to administer booster doses to most vaccine recipients in the US.
The boost campaign, which is still awaiting approval from the Food and Drug Administration, is scheduled to begin on September 20. CDC’s Advisory Committee on Immunization Practices will consider the different dose plans during a two-day meeting that begins Monday.
The finding of decreased effectiveness when delta was the dominant virus strain comes with an important caveat: the range of that estimate is highly uncertain. The researchers reported 95% confidence that the efficacy was between 26% and 84% in that period.
The observational study tracked more than 4,000 healthcare workers, first responders, and other front-line personnel at eight US locations in six states from December 2020 to August 2021. Weekly tests were conducted for Covid infection, and about 83% were vaccinated.
About two-thirds of those vaccinated had received the Pfizer Inc-BioNTech SE vaccine, 2% received Johnson & Johnson, and the remainder received the Moderna Inc. vaccine.
Overall, the vaccines were estimated to be 80% effective in preventing infection during the study period.