CD4+ T cells play vital roles in the formation of functional lung CD8+ TRM cells during viral infection and vaccination.28,39 In addition, lung-resident CD4+ helper T cells could promote humoral response and mediate heterosubtypic protection against viral infection.27,29 We next investigated the effect of the adjuvanted RBD vaccine on lung CD4+ T-cell responses after intranasal immunization. Here, CD4 is linked to viral infectious disease.