HLA-A and influenza: Based on our previous IFN-γ-ELISPOT analyses, influenza M1-specific CD4+ T-cell responses and the HLA-A*0201-restricted influenza M1 CD8+ T-cell epitope are present in frequencies between 1/10,000 and 1/1,000 [17–20], which is about the frequency of tumor-specific T cells after vaccination [8, 11].