Tdap Vaccine Effectiveness Among Adolescents: A Case-Control Study In Minnesota

Background: Despite sustained high coverage with childhood pertussis vaccines, pertussis remains poorly-controlled in the U.S.  In 2005/2006, two reduced dose acellular pertussis vaccines combined with tetanus and diphtheria toxoids (Tdap) were licensed based on immunologic bridging studies and recommended for use among adolescents and adults.   

Objectives:  To assess the effectiveness of Tdap at preventing laboratory-confirmed pertussis among adolescents 11-17 years of age.

Methods:  Laboratory-confirmed (culture or PCR) pertussis cases in persons 11-17 years of age were identified prospectively through statewide surveillance in Minnesota between October 2007 and December 2008.  Controls were selected from schools where cases occurred. After obtaining consent, demographic information, Tdap vaccination status, date of vaccination, and illness history were obtained through parental interview; persons were considered vaccinated if a Tdap vaccination record was available from a physician office, school and/or immunization registry.  Case-control sets from schools were pooled for analysis based on geographic location and date of cough onset.  Vaccine effectiveness (VE) estimates were calculated using conditional logistic regression, accounting for clustering. 

Results: A total of 99 cases and 187 controls from 36 schools were included in the analysis; median age for cases and controls was 15 and 14 years, respectively.  According to vaccination records, 9.1% of cases and 31.6% of controls received Tdap.  Overall VE against laboratory-confirmed pertussis for adolescents receiving Tdap was 72.0% (95% CI 38.0%-87.3%).  VE was higher but not significantly when vaccination occurred in the previous year compared with a year or more (75.0% (95%CI 24.2%-91.8%) vs. 68.9% (95%CI 13.9%-88.8%), respectively). 

Conclusions:  This evaluation of the U.S. pertussis vaccine recommendations suggests that Tdap is effective at preventing disease among adolescents 11-17 years of age.  Further evaluations of Tdap program effectiveness and duration of protection will be critical to maximize both direct and indirect prevention of disease.