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I was very curious to see the results of this analysis as there seems to be a suggestion that in utero exposure to inflammation may influence many health outcomes of children including autism. The issue I see with this analysis is that the follow-up for the unvaccinated group was significantly longer than the vaccinated group. Given that a significant portion of the autism diagnoses for the former occurred after 3 years of age, I don't see how anything can be concluded from the analysis done given this crucial difference between groups would influence how many diagnoses are made (the outcome of the analysis). The authors should consider adjusting for this important variable or using case-matched analysis so that both groups have the same length of follow-up.
Thank you for your comment. You are absolutely correct that the age of diagnosis is different between the two cohorts. The children in the vaccinated group seem to be diagnosed younger simply because of differential follow-up time between the vaccinated and the unvaccinated groups. The reason for this is that vaccination rates were lower in earlier years, thus the unvaccinated group are represented more in earlier birth cohorts and had the potential to reach an older age during follow-up. This is discussed in the fourth paragraph in the discussion section. For this reason we stratified the analyses by birth year to control for this issue, and the stratified results were consistent with overall results.
I read your paper with interest. One comment that comes to mind is that it seems that Table 2 suggests that the average age of diagnosis may be different among the two cohorts, in particular the children of mothers who have been vaccinated seem to be diagnosed earlier than those of mothers who have not been vaccinated. Have you computed the average age of an ASD diagnosis for both cohorts? If so, could you please share it as I did not find this information in the paper?
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