We have shown that HLA-DRB1*0101 Ifnar1−/− mice, mount an antigen-specific CD4+ T cell response against SARS-CoV-2 after DNA vaccination or viral infection (Figs. 1E and 2C), and a CD4+ T cell response to OC43 that cross-reacts with SARS-CoV-2 (Figs. 4 and S3G). Here, CD4 is linked to viral infectious disease.