Viral growth properties in A549 cells. (A) Viable virus output from A549 cells infected with clade 2.3.4 (AH05, JS08, JL09, and LN09), clade 2.3.2 (HB07, TJ09, and GD09), and clade 7 (SY08, SX09, and NX09) viruses at an MOI of 0.001. At 12, 24, 48, and 72 h postinfection, virus titers in the supernatants were determined by TCID50 assays on MDCK cells. The values are expressed as means ± standard deviations (n = 3). **, P < 0.01 between clade 2.3.4 virus-infected cells and clade 2.3.2 virus-infected cells. The dashed line represents the detection limit of the TCID50 assay. (B) A549 cells were infected with clade 2.3.4 (AH05, JS08, JL09, and LN09), clade 2.3.2 (HB07, TJ09, and GD09), and clade 7 (SY08, SX09, and NX09) viruses at an MOI of 2. At 6 h of infection, the viral NP expression levels were determined by Western blotting. β-Actin was used as a loading control.