XK and influenza: Mice immunized with H1N1 NA DNA vaccines or virus-like particles (VLPs) containing NA from 2009 pH1N1 were protected from lethal influenza A/H5N1 virus challenge [82,83,84], and ferrets immunized with inactivated H1N1, purified N1 or N1-based VLPs were partially protected against lethal H5N1 influenza challenge [85,86].