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All three COVID-19 vaccines available to Americans became less effective as the Delta variant became dominant in the U.S., with vaccine efficacy among a large group of veterans dropping between 35% and 85%, according to a new study published Thursday.
Researchers who collected the records of nearly 800,000 U.S. veterans found that as the Delta variant was emerging across American communities in March, the three vaccines were roughly equal in their ability to prevent infections.
Over the next six months, this wouldn’t be the case with Moderna’s two-dose COVID-19 vaccine — measured as 89% effective in March — was only 58% effective by the end of September, the study shows.
The study states the effectiveness of Pfizer’s two-dose vaccine fell from 87% to 45% in the same time frame.
As the Delta variant became the predominant variant in the U.S., the efficacy of Johnson & Johnson’s single-dose vaccine plunged from 86% to just 13% over those six months, the study shows.
The study — which was conducted by a team from the Public Health Institute in Oakland, the Veterans Affairs Medical Center in San Francisco, and the University of Texas Health Science Center — concluded that the Delta variant was likely the factor to look to in terms of reduced efficiency.
“The Janssen vaccine showed the greatest decline in [vaccine effectiveness against infection]. Breakthrough infections were not benign as vaccinated persons and who were subsequently infected had a higher risk of death compared to vaccinated persons who remained infection-free,” the study reads. “Importantly, vaccination still provided protection against death in infected persons, and this benefit was observed for the Moderna, Pfizer-BioNTech, and Janssen vaccines during the Delta surge, although the benefit was greater for Moderna and Pfizer-BioNTech compared to Janssen vaccines. Our findings support the conclusion that COVID-19 vaccines remain the most important tool to prevent infection and death.”
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