A nurse fills a syringe with Johnson & Johnson’s Janssen Covid-19 vaccine at a clinic on August 19 at Tournament House in Pasadena, California. (Robyn Beck/AFP/Getty Images)

Vaccine effectiveness against Covid-19 infection dropped from 91% to 66% once the Delta variant accounted for the majority of circulating virus, according to a study published Tuesday by the US Centers for Disease Control and Prevention. 

The study is in line with others from the US and around the world showing Delta’s increased tendency to cause largely minor infections among fully vaccinated people.

Still, the effectiveness of vaccines against severe disease – including hospitalization and death – has remained high against all known variants. Disease severity was not covered in the current study, however.

Instead, the new paper is the latest chapter in an ongoing study that has been following “health care personnel, first responders, and other essential and frontline workers” who receive weekly PCR tests in eight locations across six US states. The vast majority are vaccinated.

Regular testing makes it possible to capture a more complete picture of Covid-19 infections in a group, since people with mild or no symptoms may be less likely to get tested overall. Even in the official vaccine trials, efficacy was calculated against symptomatic Covid-19 – not all infections.

Still, the authors warn there is some uncertainty in these estimates, in part because they found relatively few infections in the first place. During the months when Delta was predominant, researchers found 19 infections among 488 unvaccinated people and 24 infections among 2,352 fully vaccinated people.

While waning immunity may also contribute to some degree of reduced protection, the current study was unable to show this to a significant degree.

“Notably, this analysis did not show difference over time, which suggests [vaccine] effectiveness is also decreased against Delta, independent of when you were vaccinated,” CDC Director Dr. Rochelle Walensky said last week, previewing some of the study’s early findings while making the case for booster shots down the line.

The authors of the latest paper said that, while the data shows a “moderate reduction” in the effectiveness of vaccines against infection with the Delta variant, “the sustained two-thirds reduction in infection risk underscores the continued importance and benefits of COVID-19 vaccination.”

Separate research suggests that even if a fully vaccinated person gets infected with the virus, they may be less likely to spread it.

“While we did see a reduction in the protection of the Covid-19 vaccine against the Delta variant, it’s still two-thirds reduction of risk,” lead author Ashley Fowlkes, an epidemiologist for CDC COVID-19 Emergency Response, told CNN Tuesday.

She said that researchers are getting new data every two weeks on this group – known as the HEROES-RECOVER cohort. They are planning future analyses comparing the different Covid-19 vaccines, as well as the types of symptoms experienced by vaccinated and unvaccinated people who become infected.

 “It’s still a very powerful vaccine,” Fowlkes added, especially when it comes to more severe outcomes. “But we are also looking towards continuing to use masks for a little bit longer.”

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