CD8A and influenza: NP is one of the most conserved internal influenza antigens; even the most divergent protein sequences differ by <11%.11 The presence of NP-specific T cells before exposure to virus correlated with less symptomatic, PCR-positive influenza A, during both pandemic and seasonal periods, whereas other conserved antigens were at best of lesser importance.10 Additionally, it has been recently possible to demonstrate a strong immune selection against NP CD8+ epitopes in the human influenza lineage when compared with those of the swine lineage.