Vaccines Refusers Are at Increased Risk for Contracting Pertussis

William T. Basco, Jr., MD, FAAP


September 03, 2009

Parental Refusal of Pertussis Vaccination Is Associated With an Increased Risk of Pertussis Infection in Children

Glanz JM, McClure DL, Magid DJ, et al
Pediatrics. 2009;123:1446-1451


The authors note that 21 US states allow exemption from vaccines (for school entry) based on personal beliefs, and that virtually all states allow for religious or medical exemptions.

This case-control study, conducted by Glanz and colleagues, combined results of pertussis surveillance within Kaiser Permanente Hospital's Denver-area health plan with individual chart review to determine whether "vaccine refusers" were overrepresented among identified cases of pertussis.

Participants were 2 months to 18 years of age and enrolled in the health plan during 1996-2007. The authors identified cases via laboratory results for either a positive polymerase chain reaction (PCR) or culture for Bordetella pertussis as well as a diagnosis of pertussis in the medical record. Then, each chart of a child with confirmed pertussis (by culture or PCR) was reviewed by a research assistant blind to vaccination status. The researchers then collected data around the clinical illness and outcomes for 14 days after diagnosis.

For each pertussis case, the investigators identified 4 controls from the Kaiser database who did not have pertussis diagnosis made by the index date. The controls were matched by gender and age along with time of enrollment in the health plan. Abstractors reviewed the immunization status of all participants and were blind to whether the participant was a pertussis case or a control. Any participant who had a documented refusal of pertussis vaccine for any nonmedical reason was considered a "vaccine refuser."

The authors identified 439 potential cases on the basis of pertussis diagnosis; of these, 156 participants had pertussis documented by PCR or culture and were in the study age groups. Eighteen of the cases (11.5%) were vaccine refusers. The mean age of all cases was 9 years, and the overall hospitalization rate was 6%. In the control population, only 0.5% of the children were vaccine refusers. In a secondary analysis conducted on the charts of children who had been continuously enrolled in the health plan since birth, the authors found a similar overrepresentation among a subgroup of cases and controls.

Vaccine refusal had a strong association with pertussis infection (odds ratio [OR], 22.8; 95% confidence interval, 6.7-77.5). The authors noted that children who accepted vaccines were more likely to visit the clinics for any illness (OR, 2.0), but clinicians were more likely to test for pertussis in vaccine refusers (OR, 3.2). The authors conclude that children of parents who refuse vaccines are at much greater risk for pertussis.


The authors note that differential visit rates and differential testing rates introduce biases for which they cannot control. Given that the 2 biases would work in opposite directions (underdiagnosis of pertussis in refusers because they do not come to the clinic and overdiagnosis of pertussis in refusers because they are tested more often when they do come to the clinic), it is reasonable to assume that they cancel each other out to some degree. In any case, the association is so strong that the biases would have to be large to change the overall conclusions. It also would have been interesting for investigators to have conducted regression analysis with all factors included as control variables, such as patient age, to determine the relative contribution of vaccine refusal. Even so, the authors make the point that, given the relatively low rate of pertussis in the general population, almost all pertussis cases probably result from vaccine refusal. File this article away in the "evidence for vaccine receipt" folder!



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