Two new studies released by CDC on Tuesday found that Covid-19 vaccines' effectiveness has waned somewhat, but that vaccinated people are still much less likely to develop Covid-19—and especially severe Covid-19.

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What the studies found

The first study looked at more than 4,000 frontline health care workers across eight locations in Arizona, Florida, Oregon, Texas, Utah, and Minnesota. The majority of the workers had been vaccinated with the Pfizer-BioNTech vaccine, while about one-third had received Moderna's vaccine and 2% had received Johnson & Johnson's vaccine.

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In previously released data from the study, researchers found that between Dec. 14, 2020, and April 10, 2021, the Covid-19 vaccines were about 91% effective at preventing symptomatic and asymptomatic Covid-19.

However, since the delta variant became dominant at the sites participating in the study, the effectiveness of the vaccines dropped to 66%, the researchers found. The updated study data ran through Aug. 14.

In the second study, researchers looked at 43,127 infections in Los Angeles County residents ages 16 and older. They found that, as of July 25, unvaccinated people were around five times more likely to become infected by the coronavirus—and 29 times more likely to be hospitalized with Covid-19—than those who were fully vaccinated.

'The vaccines are doing exactly what they promised us they'd do'

The researchers in the first study said that, although their data showed vaccine effectiveness decreased over time, "the sustained two-thirds reduction in infection risk underscores the continued importance and benefits of Covid-19 vaccination."

Ashley Fowlkes, an epidemiologist on CDC's Covid-19 response team and lead author of the study, said the researchers couldn't determine whether the vaccines' drop in effectiveness was due to the rise of the delta variant or simply an indication that vaccine effectiveness wanes over time.

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Fowlkes added that the drop-off "should be interpreted with caution" because the observation period in the study was short and the total number of infections was small.

"We really wanted to let people know that we were seeing a decline in the effectiveness of the vaccine in protection against any infection, symptomatic or asymptomatic, since the delta variant became dominant," she said.

"But we also want to reinforce that 66% effectiveness is a really high number," she added. "It's not 91%, but it is still a two-thirds reduction in the risk of infection among vaccinated participants."

Meanwhile, Leana Wen, an emergency physician and public health professor at George Washington University, said the Los Angeles study "tells a very compelling story of the impact of vaccination in protecting very well against severe disease and in reduction infection. It also highlights why it matters to the vaccinated if others around them are unvaccinated—the infections among the unvaccinated are spilling over and increasing the likelihood of breakthrough infections among the vaccinated."

"The vaccines are doing exactly what the promised us they'd do—they are keeping us from getting sick and dying," Barbara Ferrer, director of the Los Angeles Health Department, said, "but with the delta variant, we are seeing more transmission than we saw with the alpha variant." (Joseph, STAT News, 8/24; Banco/Cancryn, Politico, 8/24; Rabin, New York Times, 8/25; Fernandez, Axios, 8/24; Coleman, The Hill, 8/24; Achenbach/Nirappil, Washington Post, 8/24)