What is the difference between vaccine efficacy and effectiveness?

Vaccine efficacy and effectiveness measure different aspects of vaccine performance: Efficacy - measured in controlled clinical trials; compares disease incidence between vaccinated and placebo groups under ideal conditions; calculated as VE = (1 - RR) x 100% where RR is risk ratio. Trials use strict enrollment criteria, standardized administration, and close monitoring. Effectiveness - measured in real-world settings post-licensure; accounts for actual implementation including diverse populations, varying storage conditions, partial adherence, and circulating variants. Often lower than efficacy due to these factors. Both express percentage reduction in disease risk. Example: 95% efficacy means vaccinated group had 95% fewer cases than placebo. Effectiveness studies use observational designs (case-control, cohort). Both are important: efficacy for licensure decisions, effectiveness for public health policy and ongoing evaluation.