Figure 3. Results of matrix protein 2 (M2)–DNA vaccination, showing protection against divergent influenza viruses. Mice (8–10 per group) were vaccinated with DNA as described in Methods except at a dose of 100 μg/mouse. Approximately 2 weeks after the last dose of DNA, mice were challenged with 7× the 50% lethal dose (LD50) of virus and monitored for survival. A) A/PR/8 challenge: Cumulative survival rate of mice vaccinated with M-DNA or M2-DNA was significantly higher than that of mice vaccinated with B/NP-DNA (p<0.001, log rank). B) A/FM challenge: Cumulative survival rate differed significantly among groups (p = 0.041, log-rank), although in post hoc Holm-Sidak tests, pairs did not differ significantly (p≥0.05).