All participants maintained anti-tetanus concentrations of ≥0.1 IU/ml until year 10. The percentage with anti-tetanus concentrations ≥1.0 IU/ml ranged between 84.7% and 88.9% in each group at years 8.5 and 10. Anti-tetanus antibody GMCs were similar at years 8.5 and 10, but were lower than levels observed after the first booster dose