H5N1 whole-inactivated virus (WIV) immunization via intranasal and intramuscular route induced a comparable frequency of multifunctional Th1 CD4+ cells (Trondsen et al., 2015), whereas PR8 WIV strain infection induced a higher IFNγ-secreting CD4+ T cells in spleen (Bhide et al., 2019) only in intranasal route. This evidence concerns the gene CD4 and infection.