A-E Specific serum IgG antibody titers against PR8 (A), Aichi (B), and M2e (C) at 1, 2, 3, and 4 months (1, 2, 3, and 4M) after the boost (n=5). The 1M and 4M sera were used to measured virus-specific HAI titers against PR8 (D) and Aichi (E) (n=5). F ADCC surrogate assay results with five mice pooled pre-challenge sera from N 1+3+M 1M and 4M sera. M2 transiently transfected HEK293T cells were used as target cells, and PBS immunized mice sera were used as a negative control. (n = 3). Data information: In (A-F), Data are presented as mean ± SD. The statistical significance was analyzed with one-way ANOVA followed by Tukey’s test (A-E) and Student’s test (F) (* P < 0.05, ** P < 0.01, *** P < 0.001).