The CDC will continue to actively follow the HEROES-RECOVER cohort of frontline workers during the pandemic.

The three federally sanctioned COVID vaccines have lost a “moderate” amount of effectiveness against SARS-CoV-19 since the delta gene variant became the predominant strain in the United States, according to an ongoing study in frontline workers.

Investigators from the Centers for Disease Control and Prevention used data from the HEROES-RECOVER Cohorts, which includes U.S. workers most at risk of exposure to COVID-19. The latest numbers compare the vaccines’ ability to protect recipients against infection before and after the delta variant took hold, from December 2020 through August 14. It also examines whether vaccine efficacy changes with increasing time since completion of a last dose.

From Dec. 14, 2020, through April 10, 2021, the vaccines maintained 90% effectiveness in preventing symptomatic and asymptomatic infection among more than 4,000 workers across six states. From Dec. 14, 2020, to August 14, 2021, full vaccination with COVID-19 vaccines was 80% effective in preventing confirmed SARS-CoV-2 infection, “further affirming the highly protective benefit of full vaccination up to and through the most recent summer U.S. COVID-19 pandemic waves,” investigators wrote. 

Vaccine effectiveness point estimates, however, declined from 91% to 66% as the delta variant became predominant at HEROES-RECOVER study sites, reported Ashley Fowlkes, ScD, of the CDC COVID-19 Response Team. 

This trend should be interpreted with a measure of caution, because the decline in effectiveness may also be naturally declining as time since vaccination increases. And earlier comparison estimates may not be precise due to the limited observation time and few infections among participants, Fowlkes and colleagues wrote.

“Although these interim findings suggest a moderate reduction in the effectiveness of COVID-19 vaccines, the sustained two thirds reduction in infection risk underscores the continued importance and benefits of COVID-19 vaccination,” the study team concluded.

The CDC will continue to actively follow this frontline worker cohort as the pandemic continues, the investigators said.

Full findings were published in the CDC’s Morbidity and Mortality Weekly Report.