The impact of adolescent pertussis immunization, 2004―2009: lessons from Australia

To compare the impact of three strategies for delivering a booster dose of adult-formulated tetanus-diphtheria-pertussis (Tdap) vaccine to adolescents in Australia. These comprise: (i) administering Tdap to: a one-year age cohort; (ii) administering Tdap to the entire high school and to subsequent entrant cohorts; and (iii) administering Tdap to the entire high school but without continuing to immunize entrant cohorts. A series of ecologic analyses of pertussis notifications during epidemic periods in relevant age cohorts were conducted. The primary outcome measure was the incidence rate ratio (IRR), calculated by dividing pertussis incidence after the introduction of Tdap delivery programmes by pertussis incidence during the most recent pre-programme epidemic. During the epidemic period of 2008-2009, the national-level IRR among age cohorts targeted for Tdap was 0.6 (95% confidence interval, CI: 0.6-0.7), but among other age cohorts it was 1.1 (95% CI: 1.1-1.2). Only the jurisdiction that implemented strategy 2 (Western Australia) experienced sustained decreases in pertussis notifications in both adolescents and infants under 6 months of age (IRR: 0.4; 95% CI: 0.3-0.6) until 2009. If confirmed by longer experience in Australia and elsewhere, a broad school-based catch-up programme followed by immunization of school entrants may be the optimum strategy for the implementation of adolescent Tdap programmes.