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Vaccines Holding Strong Against Delta Variant

Study finds no evidence of additional breakthrough infection

If this summer’s surge in new infections has you worrying about the long-term effectiveness of COVID-19 vaccination, here’s some good news: A new study finds that the vaccines remain highly effective against the Delta variant.

Much Less Than Feared

The study, conducted at Argonne National Laboratory, analyzed time series data on COVID-19 test results by vaccination status. It concluded that the actual impact of the Delta variant on vaccine effectiveness appears much less than initially feared.

The authors examined data from seven U.S. states, five additional counties (including three in California), and the District of Columbia for the period May 15 through Sep. 15, 2021, during which the Delta variant became the dominant strain in the U.S.

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While the time series data showed a noticeable dip in vaccine effectiveness in July, the numbers began to rise again in August, and in most locales had returned to their previous high levels by early September. Since “there is no trend of systematically dropping VE [vaccine effectiveness] such as would be expected if the Delta variant were capable of strongly evading vaccine-stimulated immune response,” the authors see “no evidence that the variant causes additional breakthrough infections.” Mean vaccine effectiveness over the entire study period was 84.1 percent.

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“Still a Puzzle”

What caused such big fluctuations in July? The authors admit that’s “still a puzzle,” but theorize that it may reflect “changes in the vaccinated and unvaccinated cohort demographics, changes in social behavior, or environmental or seasonal effects.”

(Although the study doesn’t mention any specific social factors, it may be significant that the recent surge in California followed only a few weeks after state officials rescinded most COVID-19 restrictions, including mask requirements for vaccinated individuals.)

The bottom line, say the authors, is that the effectiveness of all three COVID-19 vaccines “remained relatively stable and quite well-performing, despite the dramatic concurrent rise of Delta variant prevalence.”


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