December 16, 2021

Effectiveness of mRNA vaccines against COVID-19

By: Judy Mathias
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Editor's Note

This case-controlled study by researchers from White River Junction Veterans Affairs Medical Center, White River Junction, Vermont, and Dartmouth School of Medicine, Hanover, New Hampshire, finds that mRNA vaccine effectiveness against COVID-19 decreases significantly after about 6 months.

In this analysis of 14,238 male veterans aged 65 or older with positive COVID-19 test results (cases) and 56,952 veterans with a negative test result (controls):


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  • pre-Delta vaccine effectiveness was 94% in the first month after complete vaccination and decreased to 87.9% by month 3
  • high-Delta period vaccine effectiveness was 62.0% in the first month and decreased to 57.8% by month 3
  • the decrease in vaccine effectiveness accelerated after month 4, reaching a low of about 20% in months 5 through 7.

The decrease in vaccine effectiveness could be attributable to the passage of time (ie, waning effect) and the increased transmission of the Delta variant, the authors say.

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