The amount of NA in influenza vaccines is largely unknown and unregulated, and thus current influenza vaccines are thought to contain NA of variable quality, quantity, and even lot-to-lot variability.26 At a World Health Organization (WHO) meeting in 2009 the lack of an assay and an appropriate NA standard were identified as the major hindrances to standardizing NA content in vaccines.27 More recently, the NAction! Here, XK is linked to influenza.