OBJECTIVE. The objective of this study was to determine if children who contracted pertussis infection were more likely to have parents who refused pertussis vaccinations than a similar group of children who did not develop pertussis infection.

METHODS. We conducted a case-control study of children enrolled in the Kaiser Permanente of Colorado health plan between 1996 and 2007. Each pertussis case was matched to 4 randomly selected controls. Pertussis case status and vaccination status were ascertained by medical chart review.

RESULTS. We identified 156 laboratory-confirmed pertussis cases and 595 matched controls. There were 18 (12%) pertussis vaccine refusers among the cases and 3 (0.5%) pertussis vaccine refusers among the controls. Children of parents who refused pertussis immunizations were at an increased risk for pertussis compared with children of parents who accepted vaccinations. In a secondary case-control analysis of children continuously enrolled in Kaiser Permanente of Colorado from 2 to 20 months of age, vaccine refusal was associated with a similarly increased risk of pertussis. In the entire Kaiser Permanente of Colorado pediatric population, 11% of all pertussis cases were attributed to parental vaccine refusal.

CONCLUSIONS. Children of parents who refuse pertussis immunizations are at high risk for pertussis infection relative to vaccinated children. Herd immunity does not seem to completely protect unvaccinated children from pertussis. These findings stress the need to further understand why parents refuse immunizations and to develop strategies for conveying the risks and benefits of immunizations to parents more effectively.

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