Levels and avidity of antibodies to tetanus toxoid in children aged 1–15 years in Dar es Salaam and Bagamoyo, Tanzania

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Abstract
A study was undertaken to determine the serological response in children (aged 1–15 years) immunized with diphtheria-pertussis-tetanus vaccine (DPT) alone or with a tetanus toxoid (TT) booster dose under the Expanded Programme on Immunization in Dar es Salaam and Bagamoyo, Tanzania. Using an ELISA technique, serum levels of anti-TT antibody, antibody avidity and anti-TT IgG subclasses were determined in 138 apparently healthy children. Our findings revealed that 94.7% and 98% of children aged 1–5 years in Dar es Salaam and Bagamoyo, respectively, had anti-TT antibody levels above that considered protective (≥0.1 IU/ml). Among 6–15-year-old children, 53.3% in Dar es Salaam and 55% in Bagamoyo had anti-TT antibody levels ≥ 0.1 IU/ml. The avidity index of anti-TT antibodies was high in most of the younger children, 84.2% in Dar es Salaam and 92% in Bagamoyo. Significantly fewer older children in Dar es Salaam and Bagamoyo (53.3% and 50%, respectively) had high avidity index antibodies. The predominant anti-TT IgG subclasses were IgG1 and IgG3. It is concluded that the current DPT immunization schedule provides adequate tetanus immunity for children under 5. However, about half of the older children had no protection against tetanus.