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This is a non-technical discussion of the difference between effectiveness and efficacy (two easily confused terms) in the context of vaccination. Short answer: efficacy is a measurement under ideal circumstances while effectiveness is a measurement in a “real-world” setting.

Christopher Harrison. Is vaccine effectiveness (VE) different from vaccine efficacy? Frontline Medical News, February 27, 2018. Available at https://www.pm360online.com/is-vaccine-effectiveness-ve-different-from-vaccine-efficacy/.