The CD4 and CD8 T cell IFN-γ response to H1N1pdm09, the infecting strain, increased in peripheral blood after challenge, peaking 14d post-infection (Figs 2A and 3A), then declined, reaching levels no longer significantly different from mock infected animals by 36d to 42d post-infection (Figs 2A and 3A). This evidence concerns the gene CD4 and infection.