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COVID-19 vaccine effectiveness dropped to 66% against delta, CDC finds

A nurse administers the Johnson & Johnson COVID-19 vaccine to a man at a community outreach event in Los Angeles on Aug. 22, 2021. (Image credit: REDERIC J. BROWN/AFP via Getty Images)

The delta variant has dealt a blow to COVID-19 vaccine effectiveness, which has dropped by about 25 percentage points since the variant became the dominant strain of coronavirus in the U.S., a new study among healthcare workers finds.

The study, from the Centers for Disease Control and Prevention (CDC), found that the vaccines' effectiveness against COVID-19 infections declined from 91% prior to the delta variant's emergence, to 66% after the rise of the delta variant in the summer.

Despite this "moderate reduction," health officials stressed that "the sustained two-thirds reduction in infection risk underscores the continued importance and benefits of COVID-19 vaccination," the authors wrote in the study, published Tuesday (Aug. 24) in the CDC journal Morbidity and Mortality Weekly Report (MMWR).

Related: How deadly is the coronavirus delta variant?

The study is based on information from more than 4,000 health care workers in six U.S. states (Arizona, Florida, Minnesota, Oregon, Texas and Utah), from mid-December 2020 through mid-August 2021. During the study period (both before and after the rise of the delta variant), the effectiveness of COVID-19 vaccines against infection was 80%, the study found.

Eight-three percent of healthcare workers in the study were vaccinated; 65% had received the Pfizer-BioNTech vaccine, 33% had received the Moderna vaccine and 2% had received the Johnson & Johnson vaccine.

The researchers say their finding showing a decline in vaccine effectiveness after the rise of delta should be interpreted with caution, because vaccine effectiveness may also be declining as a result of the increased time since people were vaccinated. In other words, people's immunity may have waned somewhat with the passage of time, rather than just as the result of a new variant.

The study also did not examine the severity of the infections. But a second CDC study, also published Tuesday in MMWR, did examine severity by looking at the rate of hospitalizations for COVID-19 among vaccinated and unvaccinated people in Los Angeles.

This second study, which examined more than 43,000 COVID-19 infections among L.A. residents from May 1 through July 25, 2021, found that 71.4% of infections were among unvaccinated people, 25.3% of infections were among fully vaccinated people and 3.3% of infections were among partially vaccinated people. At the end of the study period, the COVID-19 infection rate among unvaccinated people was nearly 5 times higher, and the hospitalization rate nearly 30 times higher, than the rate among fully vaccinated people.

"These infection and hospitalization rate data indicate that authorized vaccines were protective against SARS-CoV-2 infection and severe COVID-19 during a period when transmission of the delta variant was increasing," the authors wrote. "Efforts to increase COVID-19 vaccination, in coordination with other prevention strategies, are critical to preventing COVID-19-related hospitalizations and deaths."

Originally published on Live Science.  

Rachael Rettner

Rachael has been with Live Science since 2010. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.