ALDH18A1 and influenza: The high protection (about 95%) obtained after priming with PPV-PYCS and boosting with MVA-PYCS was comparable to that obtained after immunization in similar prime/boost approach but using instead two live recombinant viral vectors, i.e influenza expressing the CD8+ T cell epitope followed by VV-PYCS, that we have previously reported to be protective [10].