Neuraminidase (NA) activity assay
NA activity assays of F/98 virus and recombinant virus
rF/HAS145N were analyzed using a Neuraminidase Assay Kit
(Beyotime Institute of Biotechnology, Jiangsu, China) as previously
described (Qin et al., 2018). Each virus was calibrated with the same
TCID50 titer by diluting the virus sample. Then, 10 μL
was mixed with 70 μL detection buffer, 10 μL NA fluorogenic substrate,
and 10 μL double-distilled water. Fluorescence was monitored using a
multifunctional microplate reader (Tecan). NA activities are shown as
the fluorescence intensity above the background values for allantoic
fluid without virus.