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A Project of The Annenberg Public Policy Center

What does it mean to say a vaccine has 94% percent efficacy or higher?

This article is available in both English and Español

Efficacy is a measure of how well a vaccine performs in a clinical trial. It specifically refers to a relative reduction in infection or disease when comparing the vaccinated group to the placebo (or control) group.

For instance, both the Pfizer/BioNTech and Moderna vaccines were primarily evaluated for their ability to prevent symptomatic COVID-19, with the former having a 95% efficacy and the latter having a 94% efficacy in the clinical trial data submitted for the original authorization by the Food and Drug Administration. This means your risk of getting sick is cut by 94% or more if you are vaccinated. The final phase 3 data showed an efficacy of 91% for Pfizer/BioNTech and 93% for Moderna.

Other vaccines may report efficacy figures for infection or for moderate or severe disease.

It’s important to note that efficacy is the measure used in a clinical trial, and real-world performance, called effectiveness, could be somewhat lower. That’s measured in observational studies. Effectiveness also can differ, depending on the variant of the virus.