By Independent News Roundup
by Brenda Baletti, Ph.D. July 2, 2025 EST
A new peer-reviewed investigation into the rates of children with autism who have an intellectual disability found that those rates have risen and fallen for children born between 1992 and 2014, depending on the number of thimerosal-containing vaccines recommended for children at that time.
The study, by autism researcher Cynthia Nevison, Ph.D., is the first to systematically track trends in the rates of intellectual disability — defined as an IQ of less than 70 — among children with autism spectrum disorder (ASD). The report was published last week in Science, Public Health Policy and the Law.
By linking those trends to changes in vaccine policy, Nevison hypothesized that thimerosal, a mercury-based preservative, may play a role in modulating the severity of the disease.
To identify the trends, Nevison analyzed data in the Centers for Disease Control and Prevention’s (CDC) Autism and Developmental Disabilities Monitoring (ADDM) Network. Since 2000, ADDM has used health and education data to track autism rates among 8- and 4-year-olds at multiple sites in the U.S.
This year, the CDC released ADDM data showing that 1 in 31 (3.22%) 8-year-olds had an ASD diagnosis in 2022 — up from 1 in 36 (2.8%) in 2020. A recent JAMA Open Network study suggested that the number is closer to 1 in 33.
The CDC and press frequently argue that the rising rates are the result of better diagnosis: a broader spectrum of behavioral symptoms is recognized as part of ASD compared with years past, and providers are better trained to diagnose the illness. Some groups have celebrated the rising numbers.
However, autism advocacy organizations and some parents of children with more severe forms of autism have pushed back on that claim.
They argue that many children with autism have intellectual disabilities so severe that living independently is not possible. Attributing the rise in numbers solely to better diagnosis risks minimizing the seriousness of the disease and ignores the need to investigate its root causes, they say.
Nevison investigated these claims by tracking the rates of children diagnosed with autism who have intellectual disability, which she used as a marker for autism severity, over time. She analyzed the ADDM data since 2000, when the CDC first began collecting it, for 8-year-olds (born in 1992) to identify the proportion of ASD cases with intellectual disability.
Nevison found that the rate of intellectual disability in children with autism declined from 48% among children born in 1992 to about 38% for children born in 2000. In the next two years, the rate dropped sharply to a low of 31% among children born in 2002.
The rate remained relatively flat through 2006 before it began rising again — reaching nearly 41% for 8-year-olds born in 2014, the most recent year for which data are available.
“This trend contradicts the prevailing assumption that rising ASD prevalence is largely attributable to expanded diagnostic criteria capturing more mildly affected, higher-functioning individuals,” scientific researcher and publisher of Science, Public Health Policy and the Law, James Lyons-Weiler, Ph.D., wrote in a Substack article commenting on Nevison’s findings.
Trends linked to changes in vaccine policy
Nevison told The Defender that the trends correspond with changes in vaccine policy. She suggested the initial drop in rates among children born in the 1990s could reflect expanding diagnosis, as national attention to the disorder was increasing.
“But then what really caught my eye was that between 2000 and 2002, there was a steeper drop, and it was not accompanied by an extreme increase in the number of kids with autism overall,” she said.
Nevison noted that autism is increasingly understood to have a strong environmental component involving toxic exposures in utero and early childhood.
“These exposures can cause chronic systemic and neuro-inflammation as well as mitochondrial oxidative stress, all of which can impose metabolic stress on the developing brain, thereby altering brain development and brain functioning,” she wrote in the paper.
Children with autism have higher levels of toxic metals in their systems, including lead, mercury and aluminum. Variations in toxic metal load have been associated with the severity of autism, she wrote.
In July 1999, the CDC, the American Academy of Pediatrics and vaccine manufacturers agreed that thimerosal should be reduced or removed from childhood vaccines as a “precautionary measure.” However, they maintained there was no evidence that it posed any threat.
The CDC reported that in 2001, thimerosal was removed from all vaccines, except for multidose flu shots. Multidose flu shots, which reduce costs by reusing a single vial for multiple doses, contain thimerosal to prevent bacterial and fungal growth.
The removal of thimerosal from vaccines coincided with the steep drop in the rate of children with autism who had severe intellectual disabilities from 2000-2002. Those rates then stabilized for several years.
As CDC expanded flu shot recommendations, rates of autism with severe disability rose
In 2001, the CDC wasn’t recommending flu shots for children (or pregnant women), so those groups did not receive vaccines containing thimerosal.
However, in 2004, the CDC began recommending flu shots for pregnant women (see Figure 5 in the study) in any trimester and children ages 6 to 23 months. In 2006, the agency expanded the recommendation to children ages 6 months to 5 years.
By 2010, the CDC recommended flu vaccination for everyone older than 6 months.
Nevison’s research found that the proportion of children with intellectual disability began to increase again in 2006, the year the CDC expanded recommendations for the flu vaccine.
She said the growth was not consistent with improved diagnosis, as overall autism prevalence was also increasing. In other words, there weren’t just more children diagnosed with autism — a growing proportion of the rising number of autism cases involved intellectual disability.
The timing of policy changes and rising rates of autism with intellectual disability, combined with evidence of toxins’ role in autism severity, led Nevison to hypothesize that thimerosal could play a role in influencing the severity of autism.
“When I looked at the trends in IQ, I was kind of stunned by the patterns that emerged and felt like this was a story that needed to be told,” Nevison said.
Flu shots containing thimerosal given during pregnancy may factor into autism rates
Nevison’s study also investigated a major shift in autism prevalence nationally by race and ethnicity.
Between 2008 and 2010, Black and Hispanic children began to surpass white children in autism rates — and they also had significantly higher rates of intellectual disability.
Before that, autism prevalence in those groups had been lower, and significantly lower for Hispanic children.
Data on race and ethnicity have only been available since 2002. Since then, white children with autism diagnoses have consistently shown lower rates of intellectual disability than other groups.
For example, among children born in 2006, the rate of intellectual disability among white children diagnosed with autism was 22%, compared to 45% among Black children. Those same upward trends have continued, reaching 33% for white children and 53% for Black children born in 2014 who were diagnosed with autism and intellectual disability.
Nevison identified 2006-2008 as an inflection point, when the prevalence of autism with intellectual disability increased rapidly after a period of relative stability.
Based on those numbers, Nevison estimated that by 2014, nearly 2% of all Black children had an autism diagnosis with co-occurring intellectual disability.
She hypothesized that this inflection point aligned with the introduction of thimerosal-containing flu shots for pregnant women.
Women and children served by Medicaid are likely to follow CDC recommendations, Nevison wrote. In some states, pediatricians who treat Medicaid patients enrolled in the Vaccines for Children Program — which provides cost-free vaccines to children whose families cannot afford them — must strictly follow the CDC immunization schedule.
Nevison added:
“Similarly, women served by Medicaid are likely under pressure from their doctors to follow the CDC schedule during pregnancy. They also may be more likely to receive flu shots from thimerosal-containing multi-dose vials, while wealthier women covered by private insurance may have more opportunity to choose thimerosal-free shots from single-dose vials.
“Regardless of mercury content, the proinflammatory response to flu vaccination per se may activate the maternal immune system in ways that could be detrimental to the fetus.”
Programs such as the Children’s Health Insurance Program (CHIP), which expanded healthcare access for people just above the Medicaid threshold, increased access to regular care for lower-income people.
The program has been credited with expanding diagnosis for low-income children. Nevison noted that it also increased “well baby” visits — the primary setting for routine vaccinations.
“Thus, from the perspective of the hypothesis outlined above, CHIP could be responsible not only for improving access to the screening and diagnosis of ASD among lower-income children, but also for potentially causing more ASD among those children,” she wrote.
She also acknowledged that other factors, including diet, breastfeeding rates and higher preterm birth rates, may also put Black and Hispanic children at higher risk for autism.
At last week’s meeting of the CDC’s Advisory Committee on Immunization Practices (ACIP), the committee voted to stop recommending thimerosal-containing flu vaccines.
Dr. H. Cody Meissner, professor of pediatrics at Dartmouth’s Geisel School of Medicine, was the sole dissenting vote. “No study has ever indicated any harm from thimerosal,” he said.
Children’s Health Defense Senior Research Scientist Karl Jablonowski said Meissner based his vote on a lack of evidence, despite thimerosal’s known toxicity and its ability to cross the blood-brain barrier. Jablonowski added that Nevison’s study provides important evidence of thimerosal’s harm.
According to CNN, most flu vaccines in the U.S. come in single-dose vials that don’t contain thimerosal, but about 4% come in multidose vials that do.
During the ACIP meeting, presenter Lyn Redwood, a nurse practitioner with experience in pediatrics and family medicine, cited data showing that during the 2019-2020 flu season, over 60,000 pregnant women were vaccinated with multidose vials.
Nevison said some at the meeting dismissed the risk based on the low percentage of multidose vaccines and relatively small numbers of exposed women.
She countered that 60,000 women is still a significant number, and emphasized the importance of examining what happened to the significantly higher number of women and children who received thimerosal-containing vaccines over the last two decades.
“I don’t think you can just dismiss the gravity of this issue simply because today, less than 5% of the flu shots have thimerosal,” Nevison said.
Jablonowski added, “Our struggles with thimerosal are a testament to how much we’ve lost our sensibility when it comes to vaccines and their ingredients. We were crazy to include it, crazy to defend its inclusion, and crazy to recognize the hazard and remove it from only some.”
“Autism is not a natural disaster, it is a disaster of our own making,” he said. “We cannot un-injure children, but we can stop the injuries from continuing. Dr. Nevison’s analysis, Lyn Redwood’s ACIP presentation and the ACIP vote to recommend against intentionally added thimerosal in vaccines are necessary steps to detoxifying our future.”
Brenda Baletti, Ph.D., is a senior reporter for The Defender. She wrote and taught about capitalism and politics for 10 years in the writing program at Duke University. She holds a Ph.D. in human geography from the University of North Carolina at Chapel Hill and a master's from the University of Texas at Austin.
.svg){kind=logo}
