The comparison of all hospitalized COVID-19 patients (moderate, severe, critical course) at the time of acute disease phase with convalescent patients showed significantly decreased RBP4 plasma levels (hospitalized patients: 17.02 mg/L (11.5–22.6 mg/L) vs. convalescent patients: 21.8 mg/L (19.6–23.8 mg/L), p < 0.01) and vitamin A levels (hospitalized patients: 0.34 mg/L (0.18–0.42 mg/L) vs. convalescent patients: 0.65 mg/L (0.5–0.77 mg/L), p < 0.001; Figure 1). Here, RBP4 is linked to COVID-19.