We previously reported that the VXI-sNA is predictive of NA immunity for an N2 subtype within an H3N2 monovalent vaccine.31 Herein, we summarize the development and overall performance of the multiplexed NA potency assay for all NA subtypes (N1, N2, and B-NA) in seasonal influenza vaccines produced by a wide range of manufacturing methods. The gene discussed is XK; the disease is influenza.