The value of vaccine programme impact monitoring during the COVID-19 pandemic

Abstract

Reports of the historic vaccine development successes during the COVID-19 pandemic have highlighted two critical measures of vaccine performance—vaccine efficacy, determined by randomised controlled trials, and vaccine effectiveness, estimated from post-introduction observational studies. Both these statistics describe an individual's risk reduction after vaccination. As immunisation programmes expand globally, more estimates of a third measure of vaccine performance—vaccine impact—are needed. Vaccine impact studies estimate disease reduction in a community. These studies are typically ecological or modelling analyses that compare disease outcomes from pre-vaccine and post-vaccine introduction periods. Reductions in disease outcomes are realised through direct effects of vaccination in vaccinated people and indirect effects due to reduced transmission within a community. Sometimes other concurrent interventions or phenomena unrelated to vaccine effects, such as changes in risk behaviours or health-care practices, can reduce disease outcomes and confound assessments of vaccine impact.