Figure 2. Mice immunized with influenza A nucleoprotein and matrix DNA (A/NP+A/M DNA) are protected against lethal A/Hong Kong/156/97 (HK/156) challenge. Mice were vaccinated as in Figure 1 with A/NP+A/M DNA, with influenza B nucleoprotein DNA (B/NP+blank DNA), or with 100 mouse infectious dose (MID)50 of influenza A/Puerto Rico8/34 (A/PR/8) live virus. Sixteen days after the last dose of DNA, mice were challenged with 10,000 MID50 of HK/156/97 intranasally. a) Monitoring of morbidity by body weight loss. b) Viral titers of lung and brain homogenates. Each bar represents the result for one mouse. Dashed lines indicate detection limits. Compared to the B/NP DNA controls, lung titers were significantly reduced in the A/NP+A/M DNA group (p=0.001, analysis of variation (ANOVA)) and the A/PR/8 group (p<0.001, ANOVA). c) Survival after challenge with HK/156. d) Survival after rechallenge with 100 MID50 of HK/483 of mice primed with A/NP+A/M DNA and which had all survived the previous HK/156 infection.