CD38 and COVID-19: Finally, since the main differences in the CD4+ T-cell compartment were observed in HLA-DR, CD38 and PD1 subsets, we compared the signal intensity of these markers per cell (mean fluorescence intensity (MFI)) between INR and IR in all CD4+ cell subsets, after correcting for sex, age, COVID-19 vaccination status and seasonality (Supplementary Figure 4E).