Virus elution assay
Virus elution assay was performed as previously described (Qin et al., 2018). In brief, F/98 and rF/HAS145N viruses were diluted serially in PBS, and 50 μL aliquots of these two-fold dilutions were incubated with 50 μL 1% (v /v) chicken erythrocytes in V‐bottom microtiter plates. The plates were placed at 4°C for 30 min and then transferred to 37°C. The decrease in HA titer was monitored for 6 h, which reflects the NA‐mediated virus elution from chicken erythrocytes.