CD27 and infection: Despite numerous deregulated immune populations and their activation markers across the cohort (Figure 1), clustering based on a percentage of activated (CD69+) CD4+ T-lymphocytes, CTLA-4+ T-lymphocytes, and immature B-lymphocytes (CD19+ CD27− CD38+(bright)) revealed the best subdivision of patients based on COVID-19 severity and lung function status four to eight weeks after initial infection (Figure 2).