This review clarifies the fundamental distinction between vaccine efficacy—how well a vaccine performs under controlled, ideal clinical trial conditions—and vaccine effectiveness, which measures its real-world performance in immunization programmes. It details key observational methods used to evaluate real-world impact (such as cohort and case-control studies) while outlining common sources of analytical bias and calculation errors. Finally, it explores the concept of herd immunity, explaining how population-wide coverage indirectly protects unvaccinated individuals by dampening transmission, and emphasizing how social structures and geographic variations influence real-world disease control.
This review was part of a broader educational initiative to strengthen epidemiological methodology among infectious disease specialists in Poland. My manager, Prof. Andrzej Zieliński, invited me to co-author a review paper and deliver a plenary lecture on methods for measuring vaccine effectiveness. It was my very first experience giving an opening plenary address at the Congress of the Polish Association of Epidemiologists and Infectiologists (Polskie Towarzystwo Epidemiologów i Lekarzy Chorób Zakaźnych). I am still not entirely sure how well the technical depth landed, as audiences at opening ceremonies often expect broad, strategic overviews! 🙂