Since naive CD4+ T cells are dispensable in a memory response, our observations that naïve CD4+ T cell counts at baseline, rather than influenza specific (for all 3 vaccine strains together) CD4+CD40L+ activated T cell counts at baseline, were highly associated with the serological response to the H1N1 California strain, indicate that the CD4+ T cell immune response in sero-negative donors is indeed more a primary-like, than a memory-like, immune response against the newly emerged strain. Here, CD4 is linked to influenza.