A physically plausible incidence rate for compartmental epidemiological models

Motivated by the recent trajectory of SARS-Cov-2 new infection incidences in Germany and other European countries, this note reconsiders the need to use a non-linear incidence rate function in deterministic compartmental models for current SARS-Cov-2 epidemic modelling. Employing a homogenous contact model, it derives such function systematically using stochastic arguments. The presented result, which is relevant to modelling of proliferation of arbitrary infectious diseases, integrates well with previous analyses, in particular closes an analytical “gap” mentioned in London and Yorke (1973) and complements the stability related work on incidence rate functions of the form βI ^p S ^q seen for example in Liu, Hethcote and Levin (1987).