Estimating the Effectiveness of Tetanus-Diphtheria-Acellular Pertussis Vaccine (Tdap) for Preventing Pertussis: Evidence of Rapidly Waning Immunity and Difference in Effectiveness by Tdap Brand

Background. We estimated the vaccine effectiveness (VE) of tetanus-diphtheria-acellular pertussis vaccine (Tdap) for preventing pertussis among adolescents during a statewide outbreak of pertussis in Wisconsin during 2012. Methods. We used the population-based Wisconsin Immunization Registry (WIR) to construct a cohort of Wisconsin residents born during 1998-2000 and collect Tdap vaccination histories. Reports of laboratory-confirmed pertussis with onset during 2012 were matched to WIR clients. Incidence rate ratios (IRRs) of pertussis and Tdap VE estimates [(1 - IRR)*100%], by year of Tdap vaccine receipt and brand (Boostrix/Adacel), were estimated using Poisson regression. Results. Tdap VE decreased with increasing time since receipt, with VEs of 75.3% (95% confidence interval [CI], 55.2%-86.5%) for receipt during 2012, 68.2% (95% CI, 60.9%-74.1%) for receipt during 2011, 34.5% (95% CI, 19.9%-46.4%) for receipt during 2010, and 11.9% (95% CI, -11.1% to 30.1%) for receipt during 2009/2008; point estimates were higher among Boostrix recipients than among Adacel recipients. Among Tdap recipients, increasing time since receipt was associated with increased risk, and receipt of Boostrix (vs Adacel) was associated with decreased risk of pertussis (adjusted IRR, 0.62 [95% CI, .52-.74]). Conclusions. Our results demonstrate waning immunity following vaccination with either Tdap brand. Boostrix was more effective than Adacel in preventing pertussis in our cohort, but these findings may not be generalizable to adolescent cohorts that received different diphtheria-tetanus-acellular pertussis vaccines (DTaP) during childhood and should be further examined in studies that include childhood DTaP history.