Immunity is gained from infection and vaccination. Effective immunity that protects against infection or symptomatic infection can decay over time due to waning immunity (i.e., decay in antibodies and/or T-cells memory) and because of the evolution of the pathogen. In this talk we will review in-host models of immune system outcomes from COVID-19 infection and vaccination. We will then parameterize an age and immunity-structured population level model to track the spread of infection in the population, and the changes in population-level immunity over time. Ontario, Canada will be a case study.