A fraction of COVID-19 patients develop a severe acute respiratory distress syndrome (ARDS), requiring ICU hospitalization. In many countries, hospitals are getting overwhelmed. Identifying the patients at risk of developing ARDS will allow early treatment with immunotherapies, avoiding ICU hospitalization. Through UGent-BOF funding, we have initiated the COVID-Track study which aims at obtaining insight in COVID-19 virus-host interactions in the pulmonary tract of 10 patients. We here propose the follow-up COVID-Trace FWO project in which we will trace the viral load, presence of co-infections and disease severity in more than 100 UZ Ghent COVID-19 patients from which we already have frozen blood samples. In a few weeks, these 100 patients will be regrouped based on their disease outcome and co-infection status. For each patient subgroup, we will profile the blood cells of representative patients by single-cell proteogenomics technology. Through the larger COVIDTrack& Trace approach we will thus not only profile the pulmonary immune response (COVID-Track) but also identify cell type-specific surface markers in peripheral blood samples that correlate with the development of pathogenic lung responses, viral load, potential presence of co-infections and poor disease outcome (COVID-Trace). This will allow the design of a clinically-applicable flow cytometrybased assay to rapidly stratify COVID-19 patients in an affordable way based on easily obtainable blood samples.