Repeated vaccination against matched H3N2 influenza virus gives less protection than single vaccination in ferrets

Serological responses to HK/4801. a ELISA (left panels) and HI (right panels) assays were performed using egg-grown HK/4801 on blood samples collected from ferrets undergoing various immunization regimens. Timing of immunizations is indicated at the bottom of the charts. Red arrows: repeat vaccination immunizations; blue arrows: current-season immunizations. Animals not receiving vaccine at a time point were mock immunized with PBS injections. The time of challenge with wild-type HK/4801 is indicated with a vertical gray dashed line. Left panels: replicate 1; right panels: replicate 2. Error bars represent one standard deviation; six ferrets per group. b As above, with replicates combined. c Statistical significances of the differences between groups, as measured using a linear mixed model with repeated measures. P-values？<？0.05 are highlighted. d Blood samples were collected 4 days before viral, and the serological response to egg-adapted HK/4801 (as included in the vaccine) or to cell-grown HK/4801 (the challenge virus) was measured using HI assays (left two panels), and MN assays (right two panels). Dots represent individual ferrets; horizontal bars represent geometric mean titers (GMT); error bars represent one standard deviation. P-values (two-sided Student’s t test) are shown on the chart