HealthDay News — Undervaccinated and unvaccinated children are at increased risk for pertussis, but most cases occur in vaccinated children further away from their last vaccine dose, according to a study published online June 10 in Pediatrics.

Ousseny Zerbo, PhD, from Kaiser Permanente Northern California in Oakland, and colleagues followed children born at Kaiser Permanente Northern California between 1999 and 2016 from age 3 months. The risk for pertussis was examined by diphtheria-tetanus-acellular pertussis (DTaP) status and time since last DTaP dose. Data were included for 469,982 children ages 3 months to 11 years.

The researchers identified 738 pertussis cases. Of these, 99 cases were unvaccinated, 36 were undervaccinated, and 515 and 88 were fully vaccinated and fully vaccinated plus 1 dose, respectively. The risk for pertussis was 13 times higher among unvaccinated vs fully vaccinated children (adjusted hazard ratio [aHR], 13.53; 95% confidence interval [CI], 10.64 to 17.21) and 1.9 times higher among undervaccinated children (aHR, 1.86; 95% CI, 1.32 to 2.63). Pertussis risk was 5 times higher among vaccinated children aged 19 to <84 months at least 3 years vs less than 1 year after vaccination (aHR, 5.04; 95% CI, 1.84 to 13.80) and 2 times higher among children aged 84 to 132 months at least 6 years vs less than 3 years after vaccination (aHR, 2.32; 95% CI, 0.97 to 5.59).

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“Our results suggest that in this population, possibly in conjunction with other factors not addressed in this study, suboptimal vaccine effectiveness and waning played a major role in recent pertussis epidemics,” the authors write.

One author disclosed financial ties to the pharmaceutical industry.

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