Before revision: Transmission rates in epidemic outbreaks  vary over time depending on the societal and government responseto infections and mortality data, as evidenced in the course of the COVID-19 pandemic.
After revision: Transmission rates in epidemic outbreaks vary over time depending on the societal response. Non-pharmacological mitigation strategies such as social distancing and the adoption of protective equipment aim precisely at reducing transmission rates by reducing infectious contacts. To investigate the effects of mitigation strategies on the evolution of epidemics, nonlinear transmission rates that are influenced by the levels of infections, deaths or recoveries have been included in many variants of the classical SIR model. This class of models is particularly relevant to the course of the COVID-19 pandemic.