CD4 and infection: On day 4 post-pH1N1 virus challenge infection, significantly higher percentages of nucleoprotein (NP)-specific effector T cells (both CD3+CD4+IFN-γ+ and CD3+CD8+IFN-γ+) were observed in Prime-IIV-IIV and B-cH9/1 virus (Prime-only) immunized PBMCs as compared to those from naïve ferrets (Figure 7A,B).