In a clinical trial, adolescents who had received a booster dose of reduced dose diphtheria-tetanus-5-component acellular pertussis vaccine (Adacel, Tdap) 5 years earlier maintained increased antibody concentrations to all antigens compared with pre-vaccination values. Observed data were applied to several mathematical models designed to predict further antibody decay for pertussis antigens. A linear mixed model including a random-intercept term provided the best fit for the observed data and was used for predictions. The predicted times for sufficient antibody decay to reach pre-vaccination levels were 15.3 years (95% CI: 7.0-28.0) for pertactin, 11.0 years (5.7-18.9) for fimbriae types 2 and 3, 10.5 years (3.6-24.7) for pertussis toxoid and 9.5 years (4.2-24.6) for filamentous hemagglutinin. For at least 87% of subjects, the 10-year predicted antibody concentration was higher than the limit of quantitation (LOQ) for each pertussis antigen measured. These results support Tdap booster doses every 10 years, following the current schedule for Td vaccination.