Immunogenicity of the capsular polysaccharide-diphtheria toxoid (CRM197) conjugate vaccine. Specific pathogen free chicks (N2 strain), lacking maternal antibodies at the time of primary vaccination, were vaccinated subcutaneously at 3 and 4 weeks post-hatch (10 μg of conjugate with 10 μg CpG or with 20μg Quil A), or at 3, 4 and 5 weeks post-hatch (20 μg of conjugate with 20 μg Quil A) or received phosphate buffered saline (PBS) as a placebo. Mean serum IgG antibodies specific for capsular polysaccharide of C. jejuni strain 81-176 were assayed by ELISA and titres were expressed on a log2 scale. The limit of detection was 3.32 (log2 scale). Seroconversion [fourfold or greater increase in antibody titre (≥2 log2 units)] rates after vaccination are indicated on the right of the figure for each group. * Seroconversion rate was significantly higher than in birds receiving PBS as placebo (p < 0.05, one-tailed Fisher’s exact test).