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In spite of active measles virus (MV) vaccination strategies, reemergence continues to occur, impairing global eradication programs. The immune status against measles was evaluated in 350 vaccinated healthy Argentine children and teenagers who received a single dose of the MV Schwarz strain Lirugen vaccine (Aventis Pasteur). Sera were assessed for immunoglobulin G (IgG) antibodies by a commercial enzyme immunoassay (EIA) (Enzygnost; Behring), an in-house EIA, and neutralization EIA. Results obtained with these methods showed a marked decline in IgG level with increasing age. At 1 to 4 years of age, 84% of children had IgG antibodies above 200 mIU/ml, conventionally accepted as protective levels, whereas only 32% of older children and teenagers had antibody levels exceeding 200 mIU/ml. Moreover, the MV IgG content in the teenage group was significantly lower than the IgG antibody level of the group of younger children (P < 0.0001). In contrast, screening for IgG antibody levels to inactivated tetanus vaccine showed that, on average, 80% of this population was fully protected and that this high level of protection remained through the teenage years. This study suggests that within this population a considerable proportion of individuals had low measles antibody levels that may be insufficient to protect against reinfections or clinical disease.