The potential limitations of this analysis of the cell-mediated immune response to influenza vaccination include the absence of measures of different T cell subset frequencies that we have correlated with the cytolytic activity of CD8+ T cells Zhou and McElhaney,(2011) and other studies demonstrating that IFNγ+, CD4+, and CD8+ T cells responding to M1 and NP peptides are correlates of protection against influenza (Wilkinson et al., 2012; Sridhar et al., 2013). Here, CD4 is linked to influenza.