Figure 2.

Study design. In order to establish preexisting immunity, HLA-DR3 mice designated as OPT1, WT, and preimmune control groups were infected with sub-lethal dose of A/Hong Kong/4108/2014 (H3N2) via intranasal route (1X10⁶ PFU/mouse). The establishment of preexisting immunity to H3N2 was confirmed by seroconversion at week 4. The mice of OPT1 and WT groups received three vaccines (3 µg of H7 rHA) without adjuvant at 4 week intervals via intramuscular route. The antibody response to vaccinations was monitored from serum collected at week 14, 16 and 18. At week 20, all mice were challenged with A/Anhui/1/2013 H7N9 (10 LD50) via intranasal route (1X10⁴ PFU/mouse). Mice were monitored for clinical symptoms and weight loss for 12 d. At d 4, three mice were randomly selected from each group to assess viral lung titers.