Figure 3.

Vaccination with 2009 pandemic influenza–derived hemagglutinin (HA) immunogen mainly elicited homologous neutralization antibodies in influenza-naive mice. Six mice per group bred in specific pathogen-free environment received 4× DNA vaccine-expressing HA of pandemic vaccine, or sham DNA as mock control group. Two weeks after final inoculation, serum samples were collected and combined by group. The neutralization activities against homologous, seasonal, and highly pathogenic avian influenza viruses in both vaccinated mice (▴) and mock-control mice (Δ) were shown in serial 2-fold dilutions, triplicates were set for each dilution, and the mean value/standard error bar were used in plot. Less than 50% inhibition was considered as negative, the final dilution of serum samples able to reach ≥50% inhibition was considered as the serum sample titer. Since serum samples from 6 mice were combined for this assay, the value for each serum sample dilution represents the mean value from 6 mice.