Pertussis in California: A Tale of 2 Epidemics

Introduction: California experienced pertussis epidemics in 2010 and 2014, with more than 9000 and 11,000 reported cases, respectively. Using an ecologic study design, we compared age-specific pertussis incidence for persons ≤20 years of age in California in 2010 and 2014 to describe any changes in disease burden that may be attributable to changes in pertussis vaccination policy that occurred during interepidemic years. Methods: We evaluated California pertussis surveillance data for reported pertussis cases 20 years of age and younger with disease onset from January 1, 2010 to December 31, 2010 or from January 1, 2014 to December 31, 2014. Age-specific pertussis rates were calculated for each of the epidemic years. Characteristics of each age group were compared across epidemic years and age-specific rate ratios (RRs) and associated 95% confidence intervals (CIs) were calculated. Results: In both years, the highest disease rate was reported in infants <6 months of age; however, the incidence of pertussis declined significantly among infants <6 months of age in 2014 compared with 2010 (RR 0.58, 95% CI: 0.53-0.63). Incidence remained stable among infants ≥6 months of age, children and adolescents <12 years of age (RR 1.1, 95% CI: 0.5-1.3). Adolescents 12-18 years of age had significantly higher rates of pertussis in 2014 compared with 2010, with the highest rates observed among 14- to 16-year-olds (RR 6.5, 95% CI: 5.4-7.8). Conclusions: A 42% reduction (53%-63%) in pertussis incidence among infants <6 months of age in 2014 after the routine prenatal Tdap vaccination recommendation suggested that an increase in prenatal Tdap coverage has been having some impact on infant pertussis. The excess number of cases observed in 2014 compared with 2010 occurred almost exclusively among the 12- to 17-year-old age group. Among older children, the peak age of pertussis is shifting, consistent with aging of the acellular pertussis vaccine cohort.