The effectiveness of COVID-19 vaccines among frontline workers declined to 66% after the Delta variant became dominant, compared with 91% before it arose, according to a report from the Centers for Disease Control and Prevention.

The vaccines are still protective, the CDC said, and the finding must be interpreted with caution, as vaccine effectiveness might wane over time and the estimates of efficacy were imprecise.

“Although these interim findings suggest a moderate reduction in the effectiveness of COVID-19 vaccines in preventing infection, the sustained two-thirds reduction in infection risk underscores the continued importance and benefits of COVID-19 vaccination,” researchers wrote in the agency’s Morbidity and Mortality Weekly Report.

The findings echo earlier evidence from Israel and the U.K. suggesting COVID vaccines lost some potency in preventing infections over time as the Delta variant spread. Those conclusions helped spur the Biden administration’s plan to administer booster doses to most vaccine recipients in the U.S., which is still awaiting regulators’ sign-off. The booster campaign is set to begin Sept. 20.

The observational study tracked more than 4,000 health-care workers, first responders, and other frontline personnel in eight U.S. locations across six states from December 2020 to August 2021. They were tested weekly for COVID infection, and about 83% were vaccinated.

About two-thirds of those vaccinated had received the Pfizer Inc.-BioNTech SE shot, 2% received Johnson & Johnson’s, and the rest received the Moderna Inc. vaccine.

Overall, vaccinations were estimated to be 80% effective in preventing infection during the study period.

The finding of diminished effectiveness when Delta was the dominant virus strain comes with an important caveat: The range of that estimate is highly uncertain. Researchers reported 95% confidence that the efficacy was between 26% and 84% in that period.