Fig. 1.

Antibody responses to homologous virus after vaccination. Ferrets were immunized on days 0 and 21 with A/California/04/2009 (H1N1pdm) HA VLPs (15 μg given i.n.[IN] or 3 μg given i.m. [IM]) or commercial split vaccine A/California/07/2009 (H1N1pdm) (3 μg given i.m.). Serum was measured preimmunization (day 0), prebooster (day 21), or 14 days after booster (day 35) using HI (A) or IgG (B) or IgA (C) ELISAs using β-propiolactone-inactivated homologous virus (A/Texas/5/2009 (H1N1pdm)) as the antigen, as described in the text. Each marker represents an individual ferret given the indicated treatment; horizontal bars indicate the average titer for each group. The dashed line in panel A represents the limit of detection. Significant differences (Mann-Whitney U test) of 0.01 < P < 0.05 and P < 0.01 are indicated by * and **, respectively. With the exception of the split vaccine at day 21, titers after immunization (days 21 and 35) were all significantly greater than preimmunization titers (P < 0.05, Mann-Whitney U test), and these differences are not indicated on the charts.