BALB/c mice (three mice per group) were immunized twice intranasally with 104, 105, or 106 PFU of the PB2-RSVF virus. Two weeks after the boost immunization, IgG (A) and IgA (B, C, and D) antibodies in undiluted serum (A and B), nasal wash (C), and BAL (D) samples were detected by using an ELISA. Mice immunized with heat-inactivated RSV (HI-RSV), infected with live RSV (values for 1:100 dilution), or mock immunized with PBS served as controls. Values are expressed as the mean absorbance ± SD (n=3). Statistical significance is indicated by asterisks (*, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001).