In summary, when comparing mild vs. severe COVID-19 patients, we found that CD45- (p = 0.014; OR = 0.286, 95% CI 0.104–0.787); T-Mo CD163+/CD206- (p = 0.049; OR = 2.692, 95% CI 0.995–7.284), and CD14dim/CD33+ (p = 0.014; OR = 0.286, 95% CI 0.104–0.787) are the best options as biomarkers to discriminate between these populations. The gene discussed is CD33; the disease is COVID-19.