Fig 4. Intramuscular inoculation with a high dose of UV-inactivated virus provided partial protection against challenge with live virus.

BALB/c mice were intramuscularly inoculated with 5 × 10⁷ pfu of UV-inactivated Influenza A/WSN/1933 (UV-WSN) or UV-inactivated Influenza A/Philippines/2/1982 (UV-H3N2 Php). After 14 days, the mice were challenged intranasally with 10 LD₅₀ of live Influenza A/WSN/1933 (WSN). (A, B) Survival (A) and body weight (B) were measured for 10 days after the intranasal challenge (n = 10/group). (C–F) Lungs and sera were collected 5 days after the intranasal challenge (n = 5/group). (C) Lungs were evaluated macroscopically. (D) The viral load in the lungs was measured by plaque assay. (E) Paraffin-embedded lung sections were stained with H&E. Scale bars: 25 μm. (F) The amounts of WSN-specific (left) and H3N2 Php-specific (right) total IgG in the sera were determined by ELISA (n = 5/group). ND, not detected. **p < 0.01.