Groups of mice that were immunized via MN skin or IM injection were intranasally challenged with a lethal dose of influenza viruses, A/California (H1N1, N=3, 2× LD), A/Philippines (H3N2, N=6, 4× LD), and A/Mandarian duck (rgH5N1, N=3, 2× LD), 6 weeks after boost vaccination. (A) H1N1 virus with swine M2: Body weight changes after A/California challenge. (B) H1N1 virus with swine M2: Survival rates after A/California challenge. (C) H3N2 virus with human M2: Body weight changes after A/Philippines challenge. (D) H3N2 virus with human M2: Survival rates after A/Philippines challenge. (E) rgH5N1 virus with avian M2: Body weight changes after avian rgH5N1 A/Mandarian duck challenge. (F) rgH5N1 virus with avian M2: Survival rates after avian rgH5N1 A/Mandarian duck challenge. Body weight and survival rates were monitored for 14 days. P value indicates significant difference between M2e5x VLP (MN) and M2e5x protein (MN or IM). Error bars indicate mean ± SEM. LD, lethal dose.