Objectives To determine long-term seroprotection after serogroup C meningococcal (MenC) vaccination at the age of 9-12 years, with or without booster vaccination at the age of 13-15 years. Design Observational cohort study; follow-on from randomised study. Setting Participants were recruited from English secondary schools (in Oxfordshire and Buckinghamshire). Participants and interventions Participants were primed with MenC CRM-glycoconjugate vaccine at the age of 9-12 years in the UK routine immunisation campaign. In previous studies they were randomised at 13 to 15 years of age to receive a booster dose of MenC-CRM glycoconjugate vaccine (CRM-group) or bivalent meningococcal serogroup A/C polysaccharide vaccine (PS-group), or they received no additional doses of MenC vaccine (control group). In this follow-on study, a blood sample was obtained 11 years after primary immunisation. Of 531 individuals eligible to participate, 134 were enrolled, and 124 were included in the analysis. Main outcome measures MenC serum bactericidal antibody (SBA) geometric mean titre; proportion of participants with SBA titre ≥8 ( putative protective threshold). Results Median ages at priming, boosting and blood sampling were 10.61, 14.42 and 22.11 years, respectively. Geometric mean titres for MenC SBA were: CRM group 1373 (95% CI 954 to 1977); PS group 1024 (687 to 1526); and controls 284 (167 to 483). SBA titres =8 were present in 50/54 (92.6%) controls and 70/70 (100%) boosted individuals. Conclusions The planned introduction in the UK of an adolescent booster of MenC conjugate vaccine in 2013 is likely to provide sustained protection against MenC disease.