The main purpose of this study is to evaluate the duration of immunity after a booster dose of a MenC-CRM vaccine given to adolescents between 13 and 15 years of age. Does seroprotection persist beyond teenage years and into the early twenties? This is the age group which is most likely to carry the organism and to transmit it to other members of the population. If a booster dose of MenC vaccine given to adolescents does produce protective levels of antibody which persist into early adulthood, this would strengthen the case for such a booster to be added to the UK routine immunisation schedule, to reduce the risk of a resurgence of the disease in the future.