Histograms showing distribution of serum IgG antibodies specific for capsular polysaccharide of C. jejuni strain 81-176 in commercial broiler chicks. Chicks were vaccinated at 7 and 21 days post-hatch with capsular polysaccharide-conjugate (with CpG ODN 2007, or Addavax™ as adjuvant, or no adjuvant; antibody titres for the three vaccinated groups were pooled to graph the histograms since they did not differ significantly at any of the time points) or received PBS placebo. IgG antibody titres were determined by ELISA and analyzed using the Kruskal–Wallis (nonparametric) test because at the majority of time points, the titres were not normally distributed. In each panel, the percentage (Y-axis) of (vaccinated and PBS control) birds with particular antibody titres is plotted against their titres at that time point. A shift (to the left) in the distribution of antibody titres, consistent with decline in titres of maternal antibodies, is evident from day 7 post-hatch to day 29 (day of challenge), followed by a shift to the right after challenge. Antibody titres did not differ significantly among the four treatment groups at any of the time points. Seroconversion, a fourfold or greater (≥2 log2 units) increase in antibody titre, did not occur in any of the birds before the day of challenge (day 29 post-hatch), but 33% of vaccinated and 0% of placebo (PBS) birds seroconverted by day 38 (9 days post-challenge, a significant difference, Fisher’s exact test p < 0.01). For each time point, sera from 14 to 16 PBS (placebo) birds and sera from 40 to 43 vaccinated birds were assayed.