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There is an increased risk of invasive meningococcal disease during the teenage years. A cohort of children vaccinated with a single dose of meningococcal C protein-polysaccharide conjugate (MenC) vaccine in early childhood during the U.K. catch up campaign will enter this age group during the coming decade. The duration of protective immunity against invasive meningococcal C disease provided by this single dose regimen is uncertain.A serum bactericidal titer of <1/8 correlates with susceptibility to invasive meningococcal disease. We assessed this correlate of protection in a cohort of children ∼2 years after a single dose of vaccine.Serum bactericidal activity was assessed in 94 children (median age, 4.0 years) at a median time of 1.8 years after vaccination.Of the 94 children, 59 (63%) had a serum bactericidal titer <1/8.The data from this study add to previous evidence indicating that immunity wanes rapidly after vaccination with serogroup C meningococcal glycoconjugate vaccines in infancy and early childhood. Such observations suggest that booster doses of MenC vaccine may be needed to maintain the successful contribution this vaccine has made to child health in the United Kingdom.