Treating disease severity as an ordinal variable (Methods), multiple cell populations displayed nonlinear differences across disease severity (proliferating CD4+ and CD8+ T cells, CD8+ TTE, CD4+ TH1, CD4+ TH17, CD4+ central memory T (TCM) and IL-22+CD4+ T cells), illustrating the complex compositional changes to peripheral T cells that occur with COVID-19 (Fig. 3e and Extended Data Fig. 5b). Here, CD4 is linked to COVID-19.