Figure 2

Virulence of the recombinant viruses. The anesthetized 6-week-old C57BL/6 mice were inoculated intranasally with 300 EID₅₀ (in a 30 μL volume) of PR/8, PR/8 × KYG^NS, or PR/8 × MI^NS virus into a single nostril. Viral lung titers were measured at 3, 5, 7, and 9 dpi (A), and body weight changes were monitored daily for 14 days (B). Mortality was measured by defining an endpoint of 25% body weight loss (C). Asterisks indicate statistically significant differences between groups (*P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001; and ****P ≤ 0.0001).