Abstract

OBJECTIVE: To assess the risk of pertussis by time since vaccination in children in Minnesota and Oregon who received 5 doses of acellular pertussis vaccines (DTaP). METHODS: These cohort analyses included Minnesota and Oregon children born between 1998 and 2003 who had 5 DTaP doses recorded in state Immunization Information Systems. Immunization records and statewide pertussis surveillance data were combined. Incidence rates and risk ratios for pertussis were calculated for the 6 years after receipt of the fifth DTaP dose. RESULTS: The cohorts included 224 378 Minnesota children and 179 011 from Oregon; 458 and 89 pertussis cases were identified in Minnesota and Oregon, respectively. Pertussis incidence rates rose each year of follow-up: 15.6/100 000 (95% confidence interval [CI]: 11.1–21.4) at year 1 to 138.4/100 000 (CI: 113.3–166.9) at year 6 (Minnesota); 6.2/100 000 (CI: 3.3–10.6) in year 1 to 24.4/100 000 (CI: 15.0–37.8) in year 6 (Oregon). Risk ratios increased from 1.9 (CI: 1.3–2.9) in year 2 to 8.9 (CI: 6.0–13.0) in year 6 (Minnesota) and from 1.3 (CI: 0.6–2.8) in year 2 to 4.0 (CI: 1.9–8.4) in year 6 (Oregon). CONCLUSIONS: This evaluation reports steady increase in risk of pertussis in the years after completion of the 5-dose DTaP series. This rise is likely attributable in part to waning immunity from DTaP vaccines. Continuing to monitor disease burden and vaccine effectiveness in fully vaccinated children in coming years will be important to assess ongoing risk as additional cohorts vaccinated solely with acellular pertussis vaccines are introduced.