Objectives. We examined the impact of undetected infections, adult immunity, and waning vaccine-acquired immunity on recent age-related trends in pertussis incidence. Methods. We developed an agent-based model of pertussis transmission in Dakota County, Minnesota using case data from the Minnesota Department of Health. For outbreaks in 2004, 2008, and 2012, we fit our model to incidence in 3 children's age groups relative to adult incidence. We estimated parameters through model calibration. Results. The duration of vaccine-acquired immunity after completion of the 5-dose vaccination series decreased from 6.6 years in the 2004 model to approximately 3.0 years in the 2008 and 2012 models. Tdap waned after 2.1 years in the 2012 model. A greater percentage of adults were immune in the 2008 model than in the 2004 and 2012 models. On average, only 1 in 10 adult infections was detected, whereas 8 in 10 child infections were detected. Conclusions. The observed trends in relative pertussis incidence in Dakota County can be attributed in part to fluctuations in adult immunity and waning vaccine-acquired immunity. No single factor accounts for current pertussis trends.