Correlations with T-cell responses. A, Postvaccination interferon (IFN)-γ production by H1N1- or H3N2-stimulated CD4⁺ T cells correlated with postvaccination hemagglutination inhibition (HI), microneutralization (MN), and antibody binding capacity. B, correlations were also calculated between prevaccination IFN-γ⁺ CD4⁺ T cells and postvaccination antibody binding capacity to determine the influence of preexisting IFN-γ⁺ CD4⁺ T cells on antibody response to vaccination. The correlation coefficient and P values are presented for each comparison.