The immune response to polysaccharides and conjugate vaccines in adults is poorly understood. This study assessed meningococcal serogroup C responses after AC polysaccharide (MACP) and C conjugate (MCC) vaccine administration in young adults and explored immune memory by measuring antibody avidity. The geometric mean avidity indices (GMAIs) measured 1 month after MACP vaccination were relatively high and failed to increase significantly in the 6 months before and after a second dose of MACP/MCC. Although the GMAI of naive adults increased immediately following MCC vaccination to 215.7 (95% confidence interval, 181.0-257.1), a level similar to that seen after MACP vaccination, no further maturation in the subsequent 6 months was seen. Antibody induced by polysaccharide antigens in adults is already of relatively high avidity (compared with that in infants and toddlers) and fails to mature further, probably because both MACP and MCC predominantly stimulate memory B cells.