1. Adolescents receiving only the acellular pertussis vaccine were compared to case controls during the 2012 Washington state pertussis epidemic. Overall, Tdap vaccine effectiveness was 63.9% with a 1-year vaccine effectiveness of 73%, declining to 34% at 2 to 4 years post-vaccination.
2. The authors postulate that the lack of long term protection conferred by the Tdap vaccine has contributed to the increasing rate of pertussis cases among adolescents.
Evidence Rating Level: 2 (Good)
Study Rundown: In 1997, tetanus toxoid, reduced diphtheria toxoid and acellular pertussis (Tdap) replaced whole cell pertussis DTap vaccines. In addition, Tdap recommendations were adjusted to include vaccination for adults and teenagers in 2006. Since this change, few studies have analyzed the effectiveness of this vaccine over time. This study used the 2012 pertussis epidemic in Washington as a setting in which to explore Tdap vaccine effectiveness among adolescents. During this epidemic, the incidence of pertussis among 13 to 14 year olds was unexpectedly high, a trend not seen prior to the emergence of Tdap in 1997. This study compared pertussis cases to matched, unaffected controls. Depending on their birth year, vaccinated subjects received either a combination of whole cell and acellular pertussis vaccine or an entirely acellular series. Among adolescents receiving Tdap exclusively, vaccine effectiveness decreased by a fourth within 12 months of completing the series, by half between 1 and 2 years and finally by almost three fourths after 2 years. Comparisons between the fully acellular and mixed groups were not made for statistical reasons. The overall vaccine effectiveness for the mixed Tdap/DTap group was approximately 50%. These findings suggest that the Tdap vaccine has rapidly waning effectiveness over a relatively short period of time and imply a need to re-examine the efficacy of the current recommended Tdap vaccine schedule. Although this is one of the first major studies exploring the effectiveness of the acellular Tdap vaccine, it is limited by its lack of comparison to a mixed Tdap/DTap vaccinated group.
Study Author, Dr. Anna M. Acosta, MD, talks to 2 Minute Medicine: Epidemiologic Intelligence Service, Scientific Education and Professional Development Program Office, Meningitis and Vaccine Preventable Disease Branch, Division of Bacterial Diseases, National Center for Immunization and Respiratory Diseases.
“This article is an important read since it documents waning effectiveness of Tdap vaccine in preventing pertussis, which means clinicians’ should still consider pertussis in the differential diagnosis among those who have been vaccinated. These findings support the recent recommendation that pregnant women should get Tdap during each pregnancy to pass protective antibodies to each of their babies.”
In-Depth [case-control study]: This study included adolescents ages 11 to 19 years from 7 Washington state counties most affected by the 2012 pertussis epidemic. Three controls for every case were matched by birth year and primary provider clinic to control for clinic-based variables. Of the 836 cases included in the final analysis, 78.5% were confirmed, 11% were probably and 10.5% were suspected pertussis cases. The overall Tdap vaccine effectiveness (VE) in patients receiving an exclusively acellular series was 63.9% (95% CI: 60.3-81.8%). At 12 to 23 months postvaccination, the estimate was 54.9% (95% CI: 32.4-70.0%) and between 24 and 47 months, effectiveness was 34.2% (95% CI: -0.03-58%). When stratified by brand, Boostrix had slightly higher effectiveness than Adacel, with overlapping confidence intervals. Three quarters of both cases and controls had the full series of pertussis vaccines. The mixed Tdap/DTap group was predominantly vaccinated with Tdap and was not directly compared to the Tdap group due in part to significant differences in median age and time since vaccination between the groups (p < .0001). However, given that in the mixed vaccine group, most participants received Tdap at least 3 years before study enrollment, the Tdap duration of protection was divided into more or less than 4 years since vaccination. This resulted in a VE of 52.2% (95% CI 24.6% to 69.6%) for the former and 51.5% (95% CI 24/3% to 69%) for the latter, implying greater durability among those receiving DTap in at least part of their series.
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