Post-COVID-19 vaccine immunity seems to be waning, according to a report published yesterday (August 24) in the US Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report

The study followed more than 4,000 healthcare workers and other frontline essential workers across six states for 35 weeks, testing study participants weekly for SARS-CoV-2 infection, according to STATThe majority of the participants received one of the three vaccines in use across the US—Pfizer, Moderna, and Johnson & Johnson—although most received the Pfizer vaccine. The study started collecting data on December 14, 2020, and as of early April 2021, it showed that the vaccines were approximately 90 percent effective at preventing symptomatic and asymptomatic SARS-CoV-2 infection. 

Today’s report tracked the cohort through August 14—incorporating over four more months of data that include a time period in which the Delta variant swept the country—and found that overall, vaccine effectiveness dipped to 66 percent in the weeks when the variant accounted for at least 50 percent of SARS-CoV-2 viruses sequenced. What the study can’t answer is why effectiveness dipped in the weeks dominated by Delta. 

“What we were trying to figure out is: Is this Delta, or is this waning effectiveness?” says CDC epidemiologist and lead author of the study Ashley Fowlkes to The New York Times. “Our conclusion is that we can’t really tell.”

According to STAT, experts say the reduction in protection could be due to any number of factors, including the now-widespread Delta being able to cause more breakthrough infections than previous variants, or the possibility that immunity is waning as more time passes since the participants received their vaccines. Viral transmission is also picking up since this spring, as precautions are easing, notes STAT. The study measured the infection rate within the group of vaccinated individuals, so as viral prevalence in a community increases, it’s more likely for anyone—even vaccinated people—to be exposed to SARS-CoV-2.  

“We also want to reinforce that 66 percent effectiveness is a really high number,” Fowlkes tells the Times. “It’s not 91 percent, but it is still a two-thirds reduction in the risk of infection among vaccinated participants.”

While this research did not look at the effect of the vaccines on preventing serious infection or hospitalization, another study published in the same CDC weekly report did. That study, an analysis of infections and hospitalizations in Los Angeles County from May 1 to July 25, found that unvaccinated individuals had infection rates 4.9 times higher than vaccinated people, and hospitalization rates 29 times higher. 

“The vaccines are doing exactly what they promised us they’d do—they are keeping us from getting sick and dying, but with the delta variant, we are seeing more transmission than we saw with the alpha variant,” says Los Angeles Health Department Director Barbara Ferrer in comments to The Washington Post. 

Concerns about waning immunity have previously prompted calls for booster shots. According to a Johnson & Johnson statement released today, adding a booster of its vaccine six to eight months after the initial one-dose regimen increased antibodies to nine times higher than what was seen four weeks after the first shot.