What does it mean to say that a vaccine is effective? Health experts and scientists use a variety of terms and measures to describe how well a vaccine works.
Vaccines can be evaluated based on their ability to prevent initial infection, symptomatic disease or severe illness that can lead to hospitalization or death.
When researchers use the term efficacy, they are describing how a vaccine performs under ideal, tightly controlled conditions such as clinical trials.
Effectiveness refers to how the vaccine performs in the real world, when people are living their normal liveswithout the same controls in place.