Cases of Pertussis Due to Parents' Refusal to Vaccinate

Parents who refuse pertussis (whooping cough) vaccination for their child increase the child's risk of infection by about 20-fold. This conclusion is derived from a first-of-its-kind study of medical records within the huge Kaiser Permanente Colorado health plan (N > 430,000 members). Results of the study were published in the latest issue of Pediatrics.

Investigators extracted individual-level data from the medical charts of children (age 2 months-18 years) about documented vaccine refusal and pertussis infection during a 12-year-period (1996-2007). A total of 156 laboratory-confirmed cases of pertussis were discovered; 9 (6%) required hospitalization.* Among these 156 cases, the parents of 18 children (~12%) had refused all or nearly all of the recommended pertussis vaccinations.** Among a randomly selected, matched control population (n = 595), the vaccine refusal rate was 0.5%.

Two case-control analyses (primary and secondary) were used to determine the odds of pertussis infection as a function of vaccine refusal. (In the secondary analysis, subjects were continuously enrolled in the health plan between the ages of 2 and 20 months.)

Assessments of attributable risk suggested that all 18 cases of pertussis in unvaccinated children were due to vaccine refusal.

The investigators acknowledged that a potential bias in the study was the fact that physicians were 3 times more likely to test for pertussis in unvaccinated children who presented with a respiratory infection. But this bias was probably offset by the fact that unvaccinated children with a respiratory infection were less likely to present for evaluation than vaccinated children. 

* Notably the annual incidence of confirmed pertussis cases increased over time; more than three quarters occurred after 2001.

** The parents of 14 children had refused all routine immunizations. Most of these parents were white, older than 30 years of age, and of higher socioeconomic status.

Photo of symptomatic child with pertussis from pertussis.com.