Comparable protective efficacies against a lethal challenge by heterosubtype influenza virus A/PR/8/34(H1N1) were induced in both rNP- and RVJ1175NP-immunized mice. Ten days after the last immunization, mice were anesthetized using a pentobarbital sodium solution and challenged intranasally with a lethal dose (10×MLD50) of influenza virus A/PR/8/34(H1N1). Weight changes and the survival of the mice were monitored for three weeks (n = 11 per group, except n = 9 for the 90 μg rNP group, as two mice were killed while being anesthetized). (A) Weight changes after the viral challenge. The average percent of the initial weight is expressed as a percentage of the examined day relative to the weight prior to the challenge. (B) Percent of mice surviving after the challenge. Survival was analyzed using the Kaplan-Meier log-rank test. Significant differences in survival were compared with the PBS control groups (*, P<0.05; **, P<0.01).