Skakauskas, Vladas, and Algirdas Ambrazevičius. “An Immunity-Structured SEIRS Epidemic Model With Variable Infectivity and Spatial Heterogeneity”. Nonlinear Analysis: Modelling and Control, vol. 28, no. 4, May 2023, pp. 735-59, https://doi.org/10.15388/namc.2023.28.32230.