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 screened for significant differences in 33 blood cell types, 30 blood biochemistries, and body mass index between an infectious disease phenotype and healthy controls. We then matched critically ill COVID-19 cases with controls that exhibited mild or no symptoms after SARS-CoV-2 infection. Using data from the UK Biobank, we describe a workflow to find evidence for high neutrophil cell count and high concentrations of blood triglycerides as predictors of the immune overreaction in critical illness due to COVID-19. Based on these findings, we identified the enzyme CDK6 as a potential drug target to prevent in high risk individuals with high neutrophil cell count the immune overreaction in critical illness due to COVID-19. Three existing CDK4/6 inhibitors -- abemaciclib, ribociclib, and palbociclib -- have been approved for the treatment of breast cancer. Clinical evidence for CDK4/6 inhibitors in treating critically ill COVID-19 patients has been reported. Further clinical investigations are ongoing.