to all recent H1 vaccine strains and reacted strongly to the 1918 pandemic strain (antibodies 1009-3E04 and 1000-3E01; Fig. 3 B and Fig. 4 A, epitope-3) . These mAbs bind to past vaccine strains with higher avidity than to the pandemic H1N1. Further studies are underway to precisely identify the epitopes of all neutralizing antibodies in this study.