HA proteins have both a highly mutagenic and immunodominant head region (the outermost part of the influenza surface protein), and a stem region that remains highly conserved between various subtypes but exhibits poor immunogenicity.[40] Moreover, NA proteins have more highly conserved regions than HA proteins generally.[41] Extended delivery vaccination has the potential to generate more potent responses to the conserved HA stem region and/or NA protein, depending on the antigens used, thereby enhancing protection to a broader array of influenza strains. Here, XK is linked to influenza.