Despite the recent development of vaccines and monoclonal antibodies preventing SARS-CoV-2 infection, treating critically ill COVID-19 patients still remains a top goal. In principle, drug repurposing – the use of an already existing drug for a new indication – could provide a shortcut to a treatment. However, drug repurposing is often very speculative due to lack of clinical evidence. We report here on a methodology to find and test drug target candidates for drug repurposing. Using UK Biobank data, we matched critically ill COVID-19 cases with healthy controls and screened for significant differences in 33 blood cell types, 30 blood biochemistries, and body mass index. Significant differences in traits that have been associated with critically ill COVID-19 status in prior literature, such as alanine aminotransferase, body mass index, C-reactive protein, and neutrophil cell count, were further investigated. In-depth statistical analysis of COVID-19 associated traits and their genetics using regression modeling and propensity score stratification identified cyclin-dependent kinase 6 (CDK6) as a more promising drug target for the selective treatment of critically ill COVID-19 patients than the previously reported interleukin 6. Four existing CDK6 inhibitors -- abemaciclib, ribociclib, trilaciclib, and palbociclib -- have been approved for the treatment of breast cancer. Clinical evidence for CDK6 inhibitors in treating critically ill COVID-19 patients has been reported. Further clinical investigations are ongoing.