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. This means your risk of getting sick is cut by 94% or more if you are vaccinated.
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 will be measured in observational studies. One such study by the Centers for Disease Control and Prevention found the Pfizer/BioNTech and Moderna vaccines were 90% effective in real-world conditions, two weeks after the second dose, and 80% effective two weeks after the first dose.