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Immunization uptake in younger siblings of children with autism spectrum disorder

Posted Oct 11 2012 9:14pm

If one child has autism, the chance that a younger sibling will have autism is about 18.7%. (see the study Recurrence Risk for Autism Spectrum Disorders: A Baby Siblings Research Consortium Study and discussions here and elsewhere). Anecdotally, we hear a lot about families deciding to forgo or delay vaccines after having an autistic child. This raises a question and an opportunity for research: does delaying or stopping vaccines result in a lower risk of autism? Looking at younger siblings, one would have a population that both has a higher autism risk and a possibly higher percentage of use of alternate (including no) vaccine schedule.

A study has been published this week on this very topic: Immunization uptake in younger siblings of children with autism spectrum disorder . The answer? Immunization does not increase the risk of autism. But I get ahead of myself.

The authors divided the children into three groups

Immunization status was divided into three predefined categories: (a) Fully immunized: Children with four doses of DPTP (2, 4, 6, and 18 months) and the initial MMR dose at 12 months, (b) Partial/delayed immunizations: Children with any missing dose of DPTP or MMR at any age or a delay of 3 months or more for at least one of the doses of DPTP or MMR, and (c) Not immunized/declined: Children for whom all immunizations had been withheld as of 3 years of age.

In case you are wondering, yes, comparing groups (a) and (c) is a vaccinated/unvaccinated study design. (b) just gives more dimension to the study.

Yes, siblings of autistic children are vaccinated differently (on average) than younger siblings of non-autistic children:

MMR immunization uptake. The analysis revealed a significant group difference in MMR immunization status (Fisher’s exact test = 80.82, p < .001). Bearing in mind that the Public Health Agency of Canada recommends that children receive their initial MMR vaccine at 12 months (in contrast to the United States, where it is recommended at 12–15 months; Public Health Agency of Canada, 2006a; CDC, 2011), only 42 of the 98 (43%) younger sibs received the 12-month MMR vaccine ontime (i.e. by at least 15 months of age; see Figure 2); an additional 38 (39%) received the vaccine after 15 months of age, and 18 (18%) had not been immunized against MMR by the age of 3 years. In contrast, 88 of 98 (90%) probands received the MMR by 15 months, 9 (9.2%) were delayed, and only 1 had not been immunized by the age of 3 years. Similarly, 63 of 65 (97%) controls had completed their MMR immunization on time (i.e. only two were delayed, and none had parents who had fully declined).

Only 42% of younger siblings of autistics received the MMR ontime. 18% were not given the vaccine by age 3. Compare this to the control group, where 90% received the MMR by 15 months and 98% by age 3.

Differences were seen with the DPTP vaccine as well:

DPTP immunization uptake. A significant group difference was also found for DPTP immunization status (Fisher’s exact test = 38.95, p < .001), with just over half (55.1%) of the younger sibs having been immunized on time (31.6% were delayed, and 13.3% were not immunized by the age of 3 years; see Figure 3). The rates of DPTP uptake were higher for probands (86.7% immunized on time, 12.2% delayed, and 1% not immunized) and controls (90.8% immunized on time, 9.2% delayed, and none declined).

What did this do to autism risk for these un- and under-vaccinated younger siblings? Statistically nothing:

Of the 39 younger sibs who had completed their immunizations on time, 6 (15.7%) were diagnosed with ASD and 2 with speech-language delay (SLD). Of the 47 younger sibs for whom immunization as delayed, 15 (31.2%) received an ASD diagnosis and 2 had SLD. Of the 12 younger sibs who had not received any immunizations, 4 (33.3%) were diagnosed with ASD and 1 with SLD. Note that of those children who did not receive a diagnosis, 43.8% were fully immunized. The Fisher’s exact tests revealed no significant difference in the rates of diagnoses between immunized and nonimmunized groups for MMR (Fisher’s exact test = 5.46, p = .22), DPTP (Fisher’s exact test = 3.65, p = .44), or both (Fisher’s exact test = 4.13, p = .37), although small sample size renders these comparisons exploratory only.

And, by “statistically nothing”, I am not saying, “the calculated risk for vaccinated siblings are higher, but we can’t claim they are because the p values aren’t statistically significant”. No, I’m saying, “the calculated values are lower for vaccinated siblings.”

The authors found about 15.7% autism risk for baby siblings. Very close to the Baby Siblings study mentioned above which found 18.7% risk. The risk found for siblings with delayed vaccination was 31.2% and for unvaccinated was 33.3%. Again, these values are not statistically significant from the 15.7%.

So, when one does a vaccinated/unvaccinated study, one finds that autism risk (for familial autism) is not increased.

Since people will undoubtedly be looking for the conflicts of interests for the study authors, the COI statement is “The authors declare that there is no conflict of interest.” and their funding is “This research was funded by the Canadian Institutes of Health Research and Autism Speaks.”

Limitations include sample size and the fact that the authors relied upon parent recall for much of the data:

Parents of 22.2% (58/261) of the children provided a copy of their child’s immunization record or had it sent by their doctor; for the remaining 77.8%, status report was based on parent recall (note that this information was typically gathered at each visit, at 3- to 6-month intervals, to avoid recall bias). Due to the potential for recall bias (e.g. see Dorell et al., 2011, for bias in recall for the older children), we examined the influence of information source (card copy vs parent recall) on immunization status. No significant relationship was found for MMR (Fisher’s exact test = .38, p = .84), DPTP (Fisher’s exact test =1.71, p = .44), or “both” (Fisher’s exact test = 1.58, p = .48).

Here is the abstract:

Background: Parental concerns persist that immunization increases the risk of autism spectrum disorder, resulting in the potential for reduced uptake by parents of younger siblings of children with autism spectrum disorder (“younger sibs”).

Objective: To compare immunization uptake by parents for their younger child relative to their
older child with autism spectrum disorder (“proband”) and controls.

Design: Immunization status was obtained for 98 “younger sibs,” 98 “probands,” and 65 controls.

Results: A significant group difference emerged for overall immunization status (Fisher’s exact test = 62.70, p < .001). One or more immunizations in 59/98 younger sibs were delayed (47/98; 48%) or declined (12/98; 12.2%); immunizations were delayed in 16/98 probands (16.3%) and declined in only one. All controls were fully immunized, with only 6 (9.2%) delayed. Within the “younger sibs” group, 25/98 received an autism spectrum disorder diagnosis; 7 of whom (28%) were fully immunized. The rates of autism spectrum disorder diagnosis did not differ between immunized and nonimmunized younger sib groups, although small sample size limits interpretability of this result.

Conclusion: Parents who already have one child with autism spectrum disorder may delay or
decline immunization for their younger children, potentially placing them at increased risk of
preventable infectious diseases.

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