To determine pertussis risk by diphtheria-tetanus-acellular pertussis (DTaP) vaccination status and time since last DTaP dose.
Children born at Kaiser Permanente Northern California between 1999 and 2016 were followed from 3 months of age until they tested positive for pertussis; disenrolled from Kaiser Permanente Northern California; received the tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis, adsorbed vaccine; turned 11 years of age, or the end of the study period. DTaP vaccination status was categorized on the basis of the number of doses received in relation to the number of doses expected according to the Advisory Committee on Immunization Practice–recommended ages.
Among 469 982 children ages 3 months to 11 years, we identified 738 pertussis cases. A total of 99 cases were unvaccinated, 36 were undervaccinated, 515 were fully vaccinated, and 88 were fully vaccinated plus 1 dose. Pertussis risk was 13 times higher among unvaccinated (adjusted hazard ratio [aHR] = 13.53; 95% confidence interval [CI] 10.64–17.21) compared with fully vaccinated children and 1.9 times higher (aHR = 1.86; 95% CI 1.32–2.63) among undervaccinated children. Among vaccinated children ages 19 to <84 months, pertussis risk was 5 times higher (aHR = 5.04; 95% CI 1.84–13.80) ≥3 years vs <1 year after vaccination. Among children ages 84 to 132 months, risk was 2 times higher (aHR = 2.32; 95% CI 0.97–5.59) ≥6 years vs <3 years after vaccination.
Undervaccinated and especially unvaccinated children were at greater risk of pertussis. However, most pertussis cases occurred among children age-appropriately vaccinated who were further away from their last DTaP dose, suggesting that suboptimal vaccine effectiveness played a major role in recent pertussis epidemics.