CD4 and COVID-19: To determine the extent of cellular cross-reactivity to SARS-CoV-2 antigens, we stimulated CD4+ T cells of 60 unexposed healthy donors and 59 COVID-19 convalescents as controls (table S1) with peptide pools covering all open reading frames (ORFs) of SARS-CoV-2, referred to here as the “SARS-CoV-2 orfeome” (Fig. 1A).