Relative contributions of vaccination and previous infection to population-level SARS-CoV-2 immunity over time: a simulation modelling study

Population-level immunity to SARS-CoV-2 directly impacts the incidence of COVID-19 morbidity and mortality. Understanding how this immunity is likely to change over time in the context of future vaccination schedules and emerging SARS-CoV-2 variants is critical to inform pandemic policy. This study simulates population-level COVID-19 immunity (including relative contributions of vaccination and previous infection) in Victoria, Australia over 18 months using an agent-based model and logistic regression equations that predict immunity and waning following vaccination and/or infection. Previous infection was found to drive most immunity against infection even with ongoing regular vaccination, however a greater proportion of overall immunity against mortality was accounted for by vaccination. Although previous infection appears to be driving a substantial component of population-level COVID-19 immunity currently, improved vaccines providing longer lasting (and better sterilizing) immunity are likely to be a critical component of the future pandemic response given the risks associated with SARS-CoV-2 infection.