SIR model for assessing the impact of the advent of Omicron and mitigating measures on infection pressure and hospitalization needs

Background: On 26 November 2021, the world health organization (WHO) designated the coronavirus SARS-CoV-2 B.1.1.529 a variant of concern, named Omicron (WHO, 2021a). As of December 16, Omicron has been detected in 89 countries (WHO, 2021b). The thread posed by Omicron is highly uncertain. Methods and findings: For the analysis of the impact of Omicron on infection pressure and hospitalization needs we developed an open-source stochastic SIR (Susceptible-Infectious-Removed) fast-model for simulating the transmission in the transition stage from the prevailing variant (most often Delta) to Omicron. The model is capable to predict trajectories of infection pressure and hospitalization needs, considering (a) uncertainties for the (Omicron) parametrization, (b) pre-existing vaccination and/or partial immunity status of the population, and demographic specific aspects regarding reference hospitalization needs, (c) effects of mitigating measures including social distancing and accelerated vaccination (booster) campaigns. Conclusions: The SIR model approach yields results in fair agreement with Omicron transmission characteristics observed in South Africa and prognosis results in Europe. The equations underlying the SIR formulation allows to effectively explore the effect of Omicron parametrization on anticipated infection growth rates and hospitalization rates relative to the prevailing variant. The models are online available as open source on GitHub.