Estimating the Severity of COVID-19 Omicron Variant in the USA, India, Brazil, France, Germany, and the UK

This work evaluates the severity of COVID-19 Omicron variant in terms of the case fatality rates (CFR) via a simultaneous fitting of confirmed cases and deaths for the USA, India, Brazil, France, Germany, and the UK. The severity of Delta during the Delta-Omicron transition in the USA is also addressed in the calculations that combine Monte Carlo simulations and analytical approaches based on Gompertz functions and the least squares method. The deaths are calculated by a convolution of cases with a common gamma function for the case-to-death leg interval with a mean of 15.71±0.55\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$15.71\pm 0.55$$\end{document} days and coefficient of variation of 0.354±0.070\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.354\pm 0.070$$\end{document}, both obtained from the fitting. Linear backgrounds were included in the fitting to account for the likelihood of cases and deaths associated with other strains of the virus within the Omicron predominance. The CFRs for Omicron with 95% confidence intervals for the USA, India, Brazil, France, Germany, and the UK were 0.295 (0.154–0.436)%, 0.232 (0.134–0.331)%, 0.49 (0.27–0.71)%, 0.056 (0.028–0.084)%, 0.129 (0.074–0.184)%, and 0.168 (0.107–0.229)%, respectively. These figures are considerably lower than previous results, suggesting that the latter may have been overestimated, as the contributions of deaths from other strains of the virus were not considered. The CFR for Delta in the USA was 1.167 (1.018–1.316)%, yielding a Delta/Omicron severity ratio of 4.0 (2.5–5.4). The method can also be applied to other pandemics, contributing to the understanding of their dynamics and severity and helping decision-makers in implementing effective policies to face similar diseases around the world.