To define the capacity of a tetanus toxoid booster to reactivate infant-triggered immunity, anti-tetanus antibodies were assessed before and after boosting 162 adolescents and 219 children from Mfou (Cameroon). Among 63 adolescents with 3 recorded dose of infant DTP, 29/63 (46%) responded with a > or =4-fold increase of antibody titers, 35/63 (55%) reaching the 0.10UI/ml threshold. Response rates were slightly higher (62%) in children aged 10-11 years. Responders and non-responders only differed significantly in their baseline anti-tetanus antibodies. Thus, early life immune immaturity may limit the persistence of infant-induced immunity and subsequent boosters may be required for sustained protection.