Immunity is gained from infection and vaccination. Effective immunity can also wane over time, with antibody and/or T-cell decay, but also with pathogen evolution. We have developed mathematical models of immunity at the in-host level. Results from these models are used to inform immunity dynamics in population-level models of pathogen spread. In this talk, we will review in-host models of vaccination and infection, and we will provide an overview of immunity incorporation into mathematical epidemiological models of disease spread. Particular attention will be made to estimation of population seroprevelance with application to COVID19.