PRF1 and influenza: The top five genes of each of the two clusters (NK cell activity; NK and T‐cell activity) that were most closely correlated with influenza antibody titers had seven genes (SPON2, AKR1C3, PRF1, FCRL6, GZMA, CSTK, NKG7) which belong to the aging signature genes of IL‐7Rαlow EM CD8+ T cells.