Direct and indirect effects in vaccine efficacy and effectiveness

Area of Science:

• Epidemiology
• Biostatistics
• Vaccinology

Background:

• Established vaccine efficacy studies require comparable infection exposure between vaccinated and unvaccinated groups, as noted by Greenwood and Yule in 1915.
• The direct protective effect of a vaccine should be assessed based on protection conferred at a specific infection exposure level.

Purpose of the Study:

• To distinguish between two classes of parameters for evaluating vaccine effects.
• To differentiate parameters that control for infection exposure (direct effects) from those that include population-level impacts (indirect effects).

Main Methods:

• Conceptual framework development to distinguish vaccine effect parameters.
• Distinction of efficacy parameters (controlling for exposure) and direct effectiveness parameters (including indirect effects).

Main Results:

• Two distinct classes of parameters are proposed, moving beyond the conventional efficacy/effectiveness dichotomy.
• Efficacy parameters are defined as those controlling for infection exposure, representing direct individual protection.
• Direct effectiveness parameters are identified as encompassing both direct individual protection and indirect population-level effects.

Conclusions:

• The proposed parameter classes offer a refined approach to defining vaccine effects.
• Distinguishing between direct individual effects and population-level indirect effects is crucial for accurate vaccine assessment.
• This framework enhances the understanding of vaccine impact in both individual and population contexts.