Table 1.

Overview of vaccine efficacy and effectiveness studies.

Efficacy

Description Measures the direct protection to individuals conferred by full course of vaccination in idealized conditions Reflects the intrinsic property of vaccine (i.e., biologic efficacy) Also known as phase III

Approaches Conducted pre-licensure Randomized controlled clinical trial Typically conducted in highly restricted populations

Direct effectiveness

Description Measures the direct protection to individuals conferred by vaccination as administered in real-world ordinary conditions Typically conducted in heterogeneous populations Reflects the net benefit of vaccination as given in clinical practice Also known as phase IV

Approaches Conducted post-licensure Cohort studies and case-control studies are common study designs Typically conducted in heterogeneous populations May involve primary data collection from clinic or study populations, or linkage of surveillance databases at the individual level

Overall effectiveness

Description Measures the population-level benefit of vaccination programs that captures effects on both vaccinated and unvaccinated individuals (i.e. herd immunity) as administered in real-world ordinary conditions Typically conducted in heterogeneous populations Reflects the impact for total populations including unvaccinated by accounting for their mixing patterns and the proportion of population that is vaccinated (i.e., coverage) Also known as phase IV or impact studies

Approaches Conducted post-licensure Cohort studies and case-control studies are common study designs Ecological trend studies also commonly used Typically conducted in heterogeneous populations May involve primary data collection from clinic or study populations, or use of surveillance databases that are or are not linked at the individual level