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by Katelyn Jetelina and Andrea Tamayo, Mar 20, 2025
"Vaccines, including the MMR vaccine, do not cause autism. This has been thoroughly studied—over 25 epidemiological studies, including a fantastic 2019 analysis of more than 600,000 children in Denmark, have refuted any link. There’s also no biological plausibility: no vaccine component can alter neuron connections, and most brain changes associated with autism occur in utero, well before a child receives vaccines.
"Yet, the CDC has been tasked with running yet another study on this question. This does a disservice to parents and clinicians who need answers about autism—not reviving falsehoods that keep pulling focus from the questions we need answered.
"If vaccines don’t cause autism, what does? And are rates increasing? Let’s dig in.
Autism rates are increasing—but measuring that increase is complicated
"Autism is a spectrum of developmental differences that can present in various ways. Our understanding of autism has evolved, and so has how we diagnose it.
"A few small studies in the 1960s and ‘70s, when autism was defined much more narrowly, estimated that around 2 to 5 in 10,000 children were diagnosed with autism. By 2000, that number had risen to 66 in 10,000 U.S. children. Since then, rates estimated from a number of studies continue to climb. (See graph below.) This trend is not limited to the U.S.—other countries, including those in Scandinavia and the U.K., report similar increases.
"However, tracking trends of autism over time is challenging. Unlike conditions diagnosed with a blood test, autism is identified through behavioral assessments, which means shifts in how we define and diagnose it can influence reported rates. Research has found several factors have contributed to rising autism diagnoses:
Changes in diagnostic criteria: One study found that including milder cases accounted for 56% of the increase in California children in the 1990s. Another in Denmark found that changes in reporting practices could account for 60% of the increase from 1980 to 1991. The latest major diagnostic changes, which were made in 2013, haven’t been fully studied.
Policy changes: A 2016 study found that autism diagnoses increased in states implementing insurance mandates for autism-specific behavioral therapies.
Increased awareness: A study found that children living near a child with an autism diagnosis were more likely to be diagnosed themselves—likely because of increased parental awareness.
"These largely drive the increase in autism, but there may be more to the story. The CDC’s Autism and Developmental Disabilities Monitoring Network (ADDM), which has tracked autism rates using the same method since 2000, has also seen a rise in autism. This suggests a real rise in autism prevalence, to some extent, not just better detection.
We don’t know what’s causing autism, but we’re getting closer
"Autism is rooted in brain development. While neuroscience is still being mapped, we do know that some autistic brains show differences in neuron growth and connectivity—differences that originate in utero, when the brain is forming in the first trimester.
"So, why do some developing brains take this path while others don’t? About 85% of the risk comes from your genes. Then, environmental exposures can act as triggers, including:
Advanced parental age. Many studies suggest that both advanced paternal and maternal age are risk factors for autism, as the likelihood of spontaneous mutations in sperm and eggs increases as people get older. However, one 2016 study even suggested that younger mothers (those under 20 years old) are at increased odds of having a child with ASD.
Premature births are more likely to be diagnosed with autism.
Medications during pregnancy, such as valproic acid and thalidomide, during the first trimester of pregnancy.
Maternal inflammation. Infections, high fevers, or immune activation during pregnancy may play a role.
And others, including folic acid deficiency, obesity during pregnancy, and air pollution.
Another CDC study on vaccines and autism is a waste of resources
"Last week, reports surfaced that the CDC has been instructed to conduct yet another study on whether vaccines, particularly MMR, are linked to autism. This comes despite rigorous studies worldwide finding no connection. There is also no new safety signal that suggests something new is happening.
"It sounds like, from The Washington Post report, CDC plans to use data from its Vaccine Safety Datalink. This dataset has been used to identify real-time safety signals, such as heart inflammation with Covid-19 vaccines and adolescents. But using it to see if there’s a link between vaccines and autism presents a ton of challenges, including:
The dataset has higher vaccine coverage rates than national estimates.
Some vaccinations may not be captured by VSD.
Unvaccinated individuals in the dataset may differ from the broader unvaccinated U.S. population in a number of behaviors and medical or socioeconomic backgrounds.
Defining and diagnosing autism is complex due to the condition’s variability.
"While vaccine safety surveillance is always necessary, this particular study fuels a false narrative that something remains unresolved. It doesn’t. The timing can’t be ignored: This study was announced just as RFK Jr.—who has spent decades sowing doubt about vaccines—was appointed HHS Secretary. The autism community continues to express concern about being used as a reason to avoid vaccination.
"Most critically, this study diverts attention from the questions that matter. There are no cures for autism or treatments that reverse it, and parents need real answers—answers that could come from investing in research on its true causes rather than rehashing debunked myths. It diverts attention from needs—education, services, medical research, and compassion and understanding.
Bottom line
"Autism diagnoses have increased globally over the last 60 years. Most of this rise is due to changes in how we define and diagnose it, but even standardized tracking suggests an increase.
"What we do know is that vaccines have nothing to do with it. And every dollar spent chasing that disproven link is a dollar not spent on research that could help parents and children.
"Love,
"YLE and Andrea T"
Andrea Tamayo is a freelance health and science journalist
with a Master’s in Science Communication from the University of
California, Santa Cruz. At YLE, she also helps translate stories to
Spanish for Su Epidemiólogo Local. You can find more of her stories at andreactamayo.com.
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