Where Has All the Pertussis Gone? Pertussis Trends from 1990-2007 and the Potential Early Impact of Tdap

Background:
Despite the availability of childhood vaccines since the 1940s, pertussis remains endemic in the US with peaks in incidence every 3-5 years. The number of reported pertussis cases has been increasing since the 1980s, especially among adolescents and adults. The largest post-vaccine peak occurred in 2004 when >25,800 cases were reported nationally. In 2005, two reduced dose acellular pertussis vaccines combined with tetanus and diphtheria toxoids (Tdap) were licensed for use among adolescents and adults in the US. Understanding pertussis trends is important for evaluating the impact of Tdap.

Objectives:
To describe pertussis trends from 1990-2007 and evaluate the early direct impact of Tdap.

Methods:
We analyzed cases reported through NNDSS from 1990-2007 that met the CSTE case definition. Cases were categorized into 6 age groups (<1,1-4,5-9,10-14,15-19, and ≥20 years). To evaluate the early direct impact of Tdap while controlling for temporal variability, segmented regression was used to model trends in rate ratios; ratios were defined as disease rates in the age group routinely recommended to receive Tdap divided by the combined rate in the other age groups. Model slope coefficients from the pre-Tdap (1990-2004) period were compared to the post-Tdap (2005-2007) period.

Results:
Between 2004-2007, the incidence of pertussis decreased from 8.81/100,000 persons to 3.47/100,000 persons (-60.6%); state-specific decreases occurred in 78.4% of reporting states. Pertussis rates decreased among all age groups with the largest declines among persons 10-14 and 15-19 years old
(-77.3% and -66.6%, respectively). Slope coefficients showed a steady increase in rate ratios during the pre-Tdap period (slope=0.23, p<.0001) and a significant change to a decreasing trend post-Tdap (slope=-0.56, p=0.0005).

Conclusions:
The incidence of pertussis has decreased in the US between 2004-2007. Our evaluation suggests an impact of Tdap on this decrease. Continued monitoring of pertussis trends will be important to further evaluate the direct and indirect effects of Tdap.