BACKGROUND: California experienced pertussis epidemics in 2010 and 2014, with over 9,000 and 11,000 reported cases, respectively; more cases than observed in any years in the vaccination era. Following the 2010 epidemic, a law was passed in California that mandated one dose of Tdap vaccine for all students who entered 7th-12th grade in the 2011-2012 school year and a 7th grade requirement thereafter. In 2013, ACIP recommended that women receive Tdap vaccine at 27-36 weeks gestation during each pregnancy to maximize transplacental antibody transfer to the fetus and help protect infants <6 months of age who are too young to be fully protected through active immunization.
METHODS: We used an ecologic study design to compare pertussis incidence by age for persons <20 years of age in California in 2010 and 2014 to describe any changes in disease burden.
RESULTS: In both years, the highest disease rate was reported in infants <6 months of age, however incidence declined significantly among this age group in 2014 compared to 2010 (RR 0.58, 95% CI 0.53-0.63) and fewer hospitalizations and deaths were reported. 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). Nearly all cases with complete data had been previously immunized; the median time since vaccination was 3 years [IQR 0.85 -5 years]. Adolescents 12-18 years of age had significantly higher rates of pertussis in 2014, with the highest rates observed among 14-16 year-olds (RR 6.5, 95% CI 5.4-7.8). Nearly all cases with complete data had previously received Tdap vaccine; the median time since vaccination was 3 years [IQR 3-4 years]. Among 19 and 20 year-olds incidence remained low and did not change (RR 0.8, 95% CI 0.5-1.3).
CONCLUSIONS: The reduced incidence among infants <6 months of age suggests that efforts to encourage Tdap vaccination among pregnant women may be having some impact. The excess number of cases observed in 2014 compared to 2010 occurred almost exclusively among the 12-18 year-old age group. Beyond infancy, the peak age of pertussis is shifting upwards, consistent with aging of the cohort of children born in 1998 and after who have received only acellular pertussis vaccine. The burden of disease remained stable among older infants and children who were recently vaccinated. Surveillance data should be used to inform public health efforts and policies related to pertussis.