Pertussis outbreaks on the rise, vaccine effectiveness waning [NEJM]

Image: CC. B. Pertussis

Key study points:

1. For every year after the 5th dose of the DTaP vaccine (age 4-6), there is a 42% increased chance of getting pertussis.

Primer: Since the 1990s, the diphtheria–tetanus–acellular pertussis (DTaP) vaccine has provided coverage for children – however we still experience B. pertussis outbreaks about every 5 years. In 2010 California experienced its largest pertussis outbreak since 1958. Increasing rates could spell trouble for infants, who do not receive their first dose until 2 months of age and therefore are susceptible to infection. This study sought to analyze the waning effect of DTaP in school ages children given the increasing outbreaks.

This [case-control] study: Patients and controls were obtained from Kaiser Permanente (Northern California) member database, selected for those with PCR test results for B. pertussis from 2006-2011. Cases were those positive for pertussis and negative for parapertussis. Control groups were PCR negative for both pertussis and parapertussis, having received a 5^th DTaP dose before the PCR result. The second control group were matched to PCR-positive cases. Primary analysis was PCR-positive case patients (n=277) with PCR-negative controls (n=3318), and secondary to matched controls (n=6086). After adjustment for time, age, sex race/ethnic group and medical area, it was shown that each year after the 5^th dose of DTaP was associated with 42% increased chance of acquiring pertussis per year. Note that these were all mild-moderate cases with no hospitalizations reported. In addition, cases were statistically more likely to have received the fifth DTaP dose earlier than both PCR-negative and matched controls.

In sum: The study makes a good link between waning dose of DTaP after the fifth dose. It should be noted that the risk after 5 years of vaccination depends highly on the initial vaccine effectiveness, which can vary. This matters for calculating absolute risks. Regardless, we can conclude that the vaccine protection wanes during the 5 years after vaccination. This study has limitations in the fact that there was no unvaccinated population to compare to in order to determine true initial effectiveness. While the CDC recommends beginning the Tdap vaccines at 11 years old, it may be worthwhile to examine initiating this set of vaccinations earlier giving waning pertussis protection in school aged children.  

Click to read in NEJM

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