Hemagglutination (HI) assay. HI tests using mAb PN-SIA49 or e137 control antibody against live WS33 and VN04 viruses were performed according to standard protocols [38] . Briefly, serial dilutions of purified mAbs in PBS were performed from initial concentration of 5 mg/mL. Positive and negative control ferret sera were diluted initially 1:10 in receptor-destroying enzyme from Vibrio cholerae (Denka Seiken, Tokyo). Serial dilutions of control sera or mAbs were pre-incubated with 4 HA units of virus per well. For WS33 virus, turkey red blood cells (RBCs) were added to a final concentration of 0.5%, whereas horse RBCs were used at a 1% suspension for VN04 virus. Normal ferret serum gave a value of less than 10. Specific HI activity of mAbs was calculated as the lowest concentration of mAb that displayed HI activity.