Fact Check: Vaccine Studies

There’s an increasing number of claims about studies on vaccines and how they impact our health. Many of the claims are coming from Robert F. Kennedy, Jr., and other figures in the MAHA movement, on social media, in congressional testimony, and elsewhere. 

I know that these can be scary and confusing, and not everyone has time to go through each study. So below you’ll find a running list of these claims and what the studies really say. I hope this will be helpful when you’re processing the news or talking to your pediatrician or concerned family members.

In many cases, these studies show correlation, not causality. If you’re new here, you may find it helpful to read more about my approach to data. We will continue to update this page, so if you come across a new study that you would like to see included here, please email us at ask@parentdata.org. 

For more general vaccine questions, please check out this guide to vaccines before the age of 2 and this post on how we know vaccines are safe. 

Fact Check: Henry Ford Study is a smoking gun on vaccines and chronic disease

The claim

In Congressional testimony on September 9, 2025, a lawyer named Aaron Siri cited an unpublished study from the Henry Ford Health system. He suggested this study proved a link between vaccinations and chronic disease, including allergies, asthma, and neurodevelopmental issues, among others. He argued that the study had been withheld from publication because the authors were worried about losing their jobs if they revealed the truth. 

The facts

In this paper, the authors took 18,496 children enrolled in the health system and compared chronic disease rates for a set of 1,957 who had no vaccines to the remainder, who had at least one vaccine. They found higher rates of diagnosed issues in the vaccinated group, including asthma, autoimmune disease, eczema, and neurodevelopmental issues. The rates were two to three times as high in the vaccinated group.

This paper has the standard correlation-is-not-causality problem, but it has it to an even larger degree than most other papers. Fundamentally, the unvaccinated group here is not at all comparable to the vaccinated group.  

The first way we can see this is in differences in demographics. The two groups differ in child sex, race, prematurity, complications at birth, and birth weight. The differences are large; for example, the vaccinated group was three times as likely to be born prematurely. The groups differ on every variable the authors measure. They do not have any measures of parental income or education or any other indicators of socioeconomic status. But in every way the authors can see, the groups are different.  We would expect them to be different in unmeasured ways, too.

These differences are likely to drive many of the differences in outcomes we see. For example, prematurity is linked to many of the outcomes considered, and since the authors do not control for the level of prematurity, they are missing some ways in which the groups differ.

There is a second, very basic, problem with this study, which is that the two groups differ in the degree to which they engage with medical doctors, which means that the vaccinated group is more likely to be diagnosed with a condition, even if they are no more likely to have the condition. Children who were vaccinated in this study and had no chronic conditions had an average of seven encounters with doctors in a given year, versus just two for unexposed children without chronic conditions. Notably, when the authors limit their analysis to children with at least one medical encounter, the effects are much smaller.

The bottom line

This study isn’t convincing. The groups are too different to be remotely comparable, and it would be impossible to conclude that the differences have anything to do with vaccines. The paper is unpublished, yes, but that is probably because it is not very good, rather than anything nefarious. Even if it were published, we would still learn nothing. 

Fact Check: Vaccines increase autism by 1,135%

The claim

In an appearance on the Tucker Carlson show on June 30, 2025, Robert F. Kennedy, Jr., the Secretary of Health and Human Services, claimed that the CDC had evidence in the late 1990s that the hepatitis B vaccine caused a 1,135% increase in autism rates, and they covered it up. 

The facts

In 1999, a researcher named Thomas Vaerstraeten, who was working at the CDC at the time, presented an abstract at a meeting of the Epidemiological Intelligence Services. The abstract utilizes data from health records, which contain information on immunizations and neurological diagnoses. Based on these data, the authors of the abstract show that autism diagnosis rates are higher among children whose vaccine history indicates a higher dose of thimerosal before the age of 1 month. 

Abstracts like this one are often a first look at data, designed to point towards future work. They are not peer-reviewed and usually provide very limited detail. As far as I can tell from the limited details in the abstract, the method here is to compare the groups without any other adjustments for differences (other than gender, year of birth, and geography). 

There are a lot of other variables you’d want to account for. Perhaps most importantly, people who have more vaccines early on tend to have more well-child visits, and those well-child visits are also where other diagnoses (such as autism) are often made. 

Abstracts are typically followed by more research, which results in a peer-reviewed paper. Often, when researchers move from an abstract to a final paper, there are updates. In this case, in 2003, the research team published a more complete paper in the journal Pediatrics, which showed no link between autism and thimerosal exposure. Review data based on multiple papers shows a similar result. 

The bottom line

The most complete data on this topic does not show a link between thimerosal exposure through vaccines and neurodevelopmental issues. There was no cover-up of these results, as RFK Jr. claimed. This is the normal scientific process at work. 

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