Schematic representation of the compartmental epidemiology of the stochastic individual-based model.

The green circle denotes the IL, which increases in response to both exposure to infection and to vaccination and wanes over time (see section on the Immunological Model). The IL is tracked for each individual in the model and influences the probability of being infected (from susceptible S to exposed E) and of developing disease requiring hospitalisation (Icase) given a breakthrough infection. I_mild denotes mild symptomatic disease, I_asymp asymptomatic infection, I_hosp disease requiring hospitalisation (which is comprised of people requiring mechanical ventilation, people requiring oxygen, and people requiring neither), I_ICU disease requiring ICU admission, I_rec stepdown from ICU, and D death. ICU, intensive care unit; IL, immunity level.