Figure 2.

Schematic of the study. BALB/c mice were immunized once, twice or three times with DNA vaccine, however, the control groups were immunized three times with control plasmids. Sera were collected at the indicated times to analyse the humoral immune response and spleen cells were isolated for ELISpot assay. In parallel experiments, the remaining mice were challenged with a lethal dose of mouse-adapted viruses to detect the protective effect.