The NA gene in seasonal and avian influenza was shown to undergo differential evolution with fewer seasonal drifts compared to the HA gene.61 It has been suggested that NA should be a component of next-generation influenza vaccines since NA-inhibiting antibodies were shown to correlate with protection from severe influenza disease.41,55 In the current study, AS03-adjuvanted H5N1 vaccine promoted an expansion of the anti-NA epitope repertoire in serum antibodies (Fig. 2) along with corresponding significant increase in the NAI titers (Fig. 6). Here, XK is linked to influenza.