Objective: To evaluate the impact of the adolescent Tdap (tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis vaccine) vaccination program on pertussis trends in the United States.
Design: Retrospective analysis of nationally reported pertussis cases, January 1, 1990, through December 31, 2009.
Setting: United States.
Participants: Confirmed and probable pertussis cases. Intervention The US Tdap vaccination program.
Main outcome measure: Rate ratios of reported pertussis incidence (defined as incidence among 11- to 18-year-olds divided by the combined incidence in all other age groups) modeled through segmented regression analysis and age-specific trends in reported pertussis incidence over time.
Results: A total of 200 401 pertussis cases were reported in the United States from 1990 to 2009. Overall incidence ranged from 1.0 to 8.8 per 100,000 persons (1991 and 2004, respectively). Slope coefficients (estimated annual rate of change in rate ratios) from segmented regression showed a steady increase in pertussis incidence among adolescents 11 to 18 years old compared with all other age groups before Tdap introduction (slope = 0.22; P < .001), and a steep decreasing trend post introduction (slope = -0.48; P < .001), suggesting a direct impact of vaccination among adolescents. Indirect effects of adolescent vaccination were not observed among infants younger than 1 year.
Conclusions: Changes in pertussis incidence in the United States from 2005 to 2009 revealed a divergence between 11- to 18-year-olds and other age groups, suggesting that targeted use of Tdap among adolescents reduced disease preferentially in this age group. Increased Tdap coverage in adolescents and adults is needed to realize the full direct and indirect benefits of vaccination.