Antibody persistence and immunological priming for 2 formulations of a meningococcal group C (menC) conjugate (MCC) vaccine (containing 2 or 10 μg of menC polysaccharide) administered at 2, 3, and 4 months of age was investigated by boosting vaccine recipients at age 13-16 months or 4 years with 10 μg of unconjugated menC polysaccharide. At age 4 years, geometric mean titers (GMTs) and concentrations of menC-specific immunoglobulin G and serum bactericidal antibody (SBA) had decreased to prevaccination levels. Geometric mean avidity indices increased after the primary vaccination until age 13-16 months and then remained constant until age 4 years. One month after boosting at age 4 years, menC immunoglobulin G and SBA levels increased significantly. The postbooster SBA GMT for the 2μg vaccination (2181.2; 95% confidence interval [CI], 975.9-4875.1) was 2-fold higher than that for the 10-μg vaccination (931.6; 95% CI, 338.0-2568.1). This is the first demonstration of immunological memory at 4 years of age in children receiving MCC vaccine on the United Kingdom's 2/3/4-month immunization schedule.