Disease severity of hospitalised COVID-19 patients during the different phases of the pandemic in a primary teaching hospital in Germany

During the course of the COVID-19 pandemic, it appeared that subsequent mutations of the original virus made the strains more contagious but less virulent. Otherwise the immune status of the population improved over the same period. The aim of this study was to retrospectively investigate surrogate parameters of disease severity according to the different phases of the pandemic and their dominant strains. The phases were classified as p1 (Wuhan), p2 (alpha), p3 (delta) and p4 (omicron) according to the definitions of the Robert Koch Institute in Germany. From week 10/2020 to week 11/2022, 731 males aged 18-101, median 73 years and 699 females aged 18-100, median 76 years were admitted. The type of respiratory support (O2, non-invasive vs. invasive ventilation), discharge type, in-hospital mortality (23%/26%/14%,7%), ICU admission (20%/26%/14%/7%) differed significantly, as did the patients' immunisation status, calculated by the number of COVID-19 infections or vaccinations. To correct for the different variables (including age, sex, phase, immune status, comorbidities), multivariate analysis was performed by logistic and ordered logistic regression for the a.m. surrogate parameters. There was a highly significant inverse association between immune status and the a.m. parameters, but no association with phase regarding the dominant strains. <bold>Conclusions:</bold> There was a decrease in disease severity as assessed by surrogate parameters over the course of the pandemic, mainly related to the improved immune status of patients not the dominant strains.