Information on severity of COVID19 is crucial for preparation of healthcare sectors. We created a reference cohort of pneumonia patients from sentinel hospitals. We introduced the concept of using syndromic surveillance data to assess disease seriousness of COVID-19, directly relating the results from clinical studies and case series on COVID-19 pneumonia patients to the situation as it is observed in pneumonia patients at the beginning of seasonal influenza epidemics. We defined a reference group from 73 German sentinel hospitals of all patients diagnosed with pneumonia (codes J12-J18, primary diagnosis) that were admitted during three consecutive weeks, after the start and before the influenza epidemic in the years 2015 to 2019. We compared severity parameters that were described for COVID-19 patients (acute respiratory distress syndrome, ventilation, intensive care, case fatality) with those from the German sentinel system. First comparisons exposed a higher rate of COVID-19 patients requiring ventilation. There were more case fatalities among COVID-19 patients without comorbidities than those of influenza associated pneumonia. Hospitals should prepare for high utilisation of ventilation and intensive care resources. Although fatalities occur mostly among elderly people with chronic comorbidities, serious disease progressions do also occur among younger, healthy patients and more often than would be expected from the experience during influenza epidemics. Our approach is flexible enough to create reference cohorts, which will allow estimation of COVID-19 severity using known characteristics and outcomes.