Wednesday, May 27, 2009

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

In this month's Pediatrics is a study about parent's refusing vaccinations and an increased risk of whooping cough (pertussis) -

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

This isn't the type of study I would normally write about because to me it falls into the "duh" category. If you refuse a vaccination for a condition that is in circulation and that vaccine is even decently effective then it is obvious that you are running a greater risk of contracting the illness. However, there seems to be misinformation being reported so I thought I should address what is actually contained in the study.

The study examined records from the Kaiser Permanente Colorado (KPCO) health plan from 1996 to 2007 looking for cases of whooping cough in children of ages 2 months to 18 years. Over the 12 year period they found 439 patients with a diagnosis of pertussis and further narrowed this set down to 165 cases by limiting the group to the appropriate age age and cases that were confirmed using laboratory tests.

The vaccination status of each member of the cohort was determined by looking their medical charts and the cohort was broken up into two groups. The first group is the "vaccine refusers" which contains children whose parents refused vaccinations for "personal, non-medical" reasons. The second group contained everyone else - potentially including those who are unvaccinated for medical reasons - the study does not present any information on the composition of this group.

I think it is important to note that the study presents no data for the reason that vaccination was refused nor does it make any representations that any of these parents refused vaccinations because of fears of autism.

Returning to the 165 cases - a further 9 were excluded because the patients were either unvaccinated or partially vaccinated and the reason for the lack of vaccination was not documented.

So the final 156 case subjects were randomly matched with 624 control subject by a variety of factors. The authors excluded 29 of the control subject because the reason for lack of vaccination was not clear leaving 595 in the control group.

In the final analysis there were a total of 21 subjects in the case and control group who belonged to the "vaccine refusers" group - 18 of the subject were in the group that had whooping cough and 3 were in the group that did not.

The authors also performed a similar analysis of on a cohort of 27,748 children who were continuously enrolled in this health plan from 2 months to 20 months. In this set they matched 31 confirmed cases against 308 controls and found that there were 4 refusers in the case group and 2 who were not.

Based on the above data the authors assert that the vaccine refusers are 23 times more likely to develop whooping cough than the control group. In the secondary data set the figure was more like 19 times more likely.

The study seems to be pretty solid but there are a few potential weaknesses -

According the the CDC cases of whooping cough are primarily increasing in older children whose vaccinations are beginning to wear off - the vaccine is only effective for 6-10 years. While the study did provide information about the age brackets of the children in the case and control groups they did not provide information about how the vaccine refuser group was distributed across the age groups.

The other problem is that the size of the vaccine refusers group is very small when compared to the total number of subjects ( under 1%)

But overall the conclusions of this study are not that surprising. If you skip vaccinations you are more likely to catch what the vaccination is attempting to prevent.

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