The percentage of participants with anti-diphtheria concentrations ≥1.0 IU/ml decreased between year 8.5 and year 10 in each treatment group, and was 59.3% in the dTpa-0.133 group, 44.4% in the dTpa-0.3 group, and 33.3% in the dTpa-0.5 group at year 10 (Table 2). Anti-diphtheria GMCs also decreased over time but remained higher than levels prior to the first dTpa dose