Fig. 1

Immunization schedule and fusion gene constructs. a, immunization schedule, the mice were immunized three times at four weeks intervals through IM and IN routes, samples were collected at two weeks after final immunization, mice were challenged at four weeks after final immunization, partial mice were kept for the long-lasting antibody test in each group. b, c diagrams of 4M2e and HA stalk domain fusion gene. b, to generate constructs HA stalk domain, the sequence corresponding HA globular head domain (amino acid 59–292) were replaced by a flexible linker sequence encoding (GGGGGS)4. c, 4M2e, four M2e copies in tandem repeat form. The GPI anchor was fused to the ends of the HA stalk domain and 4M2e coding gene