CD38 and COVID-19: The antigen‐reactive CD4+ T cells of individuals with COVID‐19 were significantly more activated (indicated by co‐expression of CD38 and HLA‐DR; Figure 5d) and showed increased frequencies of PD‐1+, CD57+, TIGIT+ and KLRG1+ cells than bulk CD4+ T cells, while the CD127+ frequency was reduced (Figure 5e and Supplementary figure 13a, d).