The reviewer makes a good about the potential for false correlation between the count of immunizations and autism. Also the children with diagnosed autism potentially were more likely to have early healthcare visits, and to follow their pediatrician's advice- including getting shots. So it is more likely that the reverse is true- that autism, diagnosed or not, causes more immunizations.

All that said, retrospective cohort data is hard to work with, but I've seen poorly setup randomizations that are worse. Large observational data offers the chance to stage a post-hoc randomization- if the potential biases can be factored in.

I've thought of trying to replicate this study's finding, as I can get access to potentially several state's Medicaid data. But there would need to be a control for poverty, compliance with medical advice, and for total number of visits prior to autism diagnosis- at a minimum

March 17, 2025

Dear Editor:

In the review of the recent vaccine study (1) it is stated that important limitations were omitted in the paper (about frequency of visits to a doctor, etc). However, the reviewer does not provide any references to support the claims about the supposed limitations.

The reviewer also states that “observational studies will always be confounded” and that “we need large randomized trials…” Yet later in his review he cites observational studies to support his attack on “one of the worst fraudsters in medical history.”

Apparently, the reviewer is unaware that good observational studies tend to arrive at the same conclusions as good randomized studies (2). Moreover, observational studies may be more advantageous versus randomized studies when investigating potential unintended effects. (3).


1. Gotzsche P. Open peer review of: Vaccination and neurodevelopmental disorders: a study of nine-year-old children enrolled in Medicaid. Journal of the Academy of Public Health 2025; Mar 3: DOI: https://doi.org/10.70542/rcj-japh-art-1li8wxw

2. Mariani AW, Pêgo-Fernandes PM. Observational studies: why are they so important? Sao Paulo Med J. 2014;132(1):1-2.

3. Bosdriesz JR, Stel VS, van Diepen M, Meuleman Y, Dekker FW, Zoccali C, Jager KJ. Evidence-based medicine-When observational studies are better than randomized controlled trials. Nephrology 2020; 25(10):737-743.


John Hart, DC, MHSc
Greenville, South Carolina