The Waning Protection of the Pertussis Vaccine

Waning Tdap Effectiveness in Adolescents

Klein NP, Bartlett J, Fireman B, Baxter R
Pediatrics. 2016;137:1-9

Study Summary

Since 2006, vaccine recommendations in the United States have included a booster dose of Tdap, in part because of the shift from whole-cell to acellular pertussis vaccine in the 1990s. This is a follow-up study of a cohort of adolescents who received only acellular pertussis vaccine for the primary series and were among the first to receive the Tdap booster dose in early adolescence. The adolescents in the study were members of Kaiser Permanente of Northern California and had received the Tdap vaccine after age 10 years. Products manufactured by different companies were used during the study for both the acellular pertussis vaccine and the booster vaccine. The outcome of interest was vaccine effectiveness, which was calculated for several windows of time after receipt of Tdap: from 8 to 365 days (year 1) after vaccination, year 2, year 3, and year 4 and beyond. Cases were those with positive pertussis polymerase chain reaction tests.

Outbreaks of pertussis affected health plan members in 2010 and 2014, producing 1207 pertussis cases among more than 279,000 eligible adolescents. During the 2010 epidemic, the age of peak pertussis incidence was 10-11 years. During the 2014 epidemic, the age of peak pertussis incidence was 15 years. The calculated vaccine effectiveness was 68.8% (95% confidence interval, 59.7%-75.9%) during year 1. During year 2, the vaccine effectiveness was 56.9%, declining to 25.2% during year 3. For year 4 and beyond, the vaccine effectiveness was only 8.9%. The 95% confidence intervals for years 1 and 2 overlap slightly, as do the confidence intervals for years 2 and 3, and likewise for years 3 and 4. However, the confidence intervals for year 1 and year 2 do not overlap with those of year 4 and beyond, demonstrating a statistically significant decline, as well as the marked clinical decline in vaccine effectiveness. The investigators concluded that Tdap provides only moderate protection in the first year after vaccination, and that effectiveness wanes rapidly thereafter at a loss of approximately 35% annually. They questioned whether routine administration of Tdap will be effective given the relatively short effectiveness of the vaccine unless it is administered in anticipation of outbreaks.

Viewpoint

A strength of this longitudinal cohort study was the ability to assess all of the medical care received by health plan members over the course of study. The occurrence of two epidemics of pertussis allowed the investigators to calculate and identify changes in pertussis vaccine effectiveness. The cohort studied was among the first to have received only acellular pertussis vaccines during their preschool vaccinations, and the ages of teens who experienced the highest incidence of pertussis in each of the outbreaks (10 years in 2010 and 15 years in 2015) corresponded very closely to the aging of the acellular-only cohort. The investigators acknowledged that it is difficult to ascertain the relative contribution of vaccine type to the increased rate of pertussis infection—whether it was the acellular vaccines in the primary series or the Tdap booster—but they are very concerned that future outbreaks will occur among older individuals as the acellular-only cohort ages further and becomes more susceptible owing to waning effectiveness of acellular vaccines in general. This concern must be counterbalanced with the very real reasons why we moved away from whole-cell pertussis vaccine—primarily vaccine side effects in the first week after vaccination. These data suggest that the epidemiology of pertussis outbreaks needs to be continually followed to ascertain whether the current vaccination approach is adequate.

Abstract

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