CRP and COVID-19: As expected, in our study, low leucocyte count and elevated CRP levels were highly indicative of a primary case, as was higher mean corpuscular hemoglobin value.24,25 The need for oxygen therapy was a good indicator for primary cases, in line with current knowledge and treatment recommendations.7,26 Hypoxic respiratory disease has been mentioned as an indicator for disease severity in the context of vaccine effectiveness and as a differentiation criterion for incidental SARS-CoV-2 infection without clinically relevant COVID-19.27,28