Efficacy refers to how well a vaccine performs under ideal conditions.
Vaccine efficacy can only be determined in clinical trials, and these trials typically have very specific criteria for participants. The criteria nearly always include age requirements and often have sex, gender, race, or ethnicity requirements as well. Other potential criteria could relate to having or not having certain underlying conditions, having a certain severity of a particular disease, living in a particular geographical area, living in a rural or urban or suburban area or having public, private or no insurance, among other things.