Under Robert F. Kennedy Jr., a longtime anti-vaccine advocate who is now Health and Human Services secretary, the Centers for Disease Control and Prevention revised its website to say that its previous statement that “vaccines do not cause autism” is “not an evidence-based claim.” But it’s the revised website that misleads about vaccines.
On Nov. 19, the CDC replaced its webpage on autism and vaccines with a new one that leans into the discredited idea that vaccines might cause autism. Numerous rigorous studies have repeatedly failed to identify any link between vaccination and autism.
“The claim ‘vaccines do not cause autism’ is not an evidence-based claim because studies have not ruled out the possibility that infant vaccines cause autism,” the webpage reads. “Studies supporting a link have been ignored by health authorities.”
A prominent website subheading still reads “Vaccines do not cause Autism” but now includes an asterisk. A footnote explains that the phrase was not removed due to an agreement with Republican Sen. Bill Cassidy of Louisiana. A physician who is a strong supporter of vaccines, Cassidy decided to back Kennedy’s nomination as HHS secretary only after Kennedy promised not to take down CDC statements that say vaccines do not cause autism, among other concessions to protect vaccination. Cassidy’s vote in a Senate committee was needed to move Kennedy’s nomination to the full Senate.
The webpage begins by noting the increasing number of vaccinations given to infants since 1986 and notes a correlation to the rising prevalence of autism. Although it acknowledges that correlation is not causation, the page cites a 2014 study by a researcher with ties to anti-vaccine groups that claimed to have found a correlation between aluminum in vaccines and autism. Other, stronger research — which the revised CDC webpage misinterprets — contradicts the idea that aluminum in vaccines is associated with autism.
The webpage goes on to argue that there are “no studies” that show that seven vaccines given before the age of 1 do not cause autism. It also alleges that the very robust evidence that the measles, mumps and rubella, or MMR, vaccine doesn’t cause autism is weak, emphasizing that the studies are all retrospective epidemiological studies, which, unlike randomized controlled trials, “cannot prove causation.”
As we’ll explain, these arguments are misleading.
“This is absolute insanity and a bizarre moving of the goalposts,” David S. Mandell, a psychiatry professor at the University of Pennsylvania Perelman School of Medicine and director of the Penn Center for Mental Health, told us via email. “As any scientist knows, you can’t ‘prove’ the lack of association. You conduct related studies, over and over, until the bulk of evidence finds no association.”
He added: The “CDC page is the equivalent of ‘you haven’t proven that ghosts don’t exist’ or perhaps more to the point, ‘you haven’t proven that driving during pregnancy doesn’t cause autism, so pregnant women should stop driving.’”
As Children’s Hospital of Philadelphia pediatrician and vaccine expert Dr. Paul Offit told us in a phone interview and has explained in a Substack post, scientists can’t ever prove a negative. In this way, he said, the webpage is taking advantage of a technicality of the scientific method, even though overwhelming evidence shows no link between vaccines and autism.
We emailed the CDC asking for more detail on the webpage’s claims about vaccines and autism, as well as what prompted the revision. HHS Communications Director Andrew Nixon replied, “This is a common-sense update that brings CDC’s website in line with our commitment to transparency and gold standard science.” He went on to reiterate the claims made on the webpage before adding that the updates are part of a “broader effort to ensure all public-facing information reflects ongoing scientific inquiry.”
In a post on X on Nov. 20, Cassidy addressed the website change. “What parents need to hear right now is vaccines for measles, polio, hepatitis B and other childhood diseases are safe and effective and will not cause autism. Any statement to the contrary is wrong, irresponsible, and actively makes Americans sicker,” he wrote, noting several recent outbreaks of vaccine-preventable diseases. “Redirecting attention to factors we definitely know DO NOT cause autism denies families the answers they deserve,” he added (emphasis is his).
Former CDC leaders and current agency officials have said that career staff were not involved in or aware of the CDC update, according to reporting by the Washington Post and STAT.
Infant Vaccine Sleight of Hand
A core claim of the new webpage is that it hasn’t been shown that seven infant vaccines (DTaP, hepatitis B, polio, pneumococcus, Hib, rotavirus and influenza) do not cause autism. This is misleading, as most of these vaccines have been studied before in some way for autism — just not necessarily on their own.
“This is a reversal of the burden of proof. This is not how science works,” Anders Hviid, the head of the epidemiology research department at the Statens Serum Institut in Denmark, told us in an email.
He noted that claims about infant vaccines and autism have focused on the presence of small amounts of aluminum, which is added to boost the immune response. Aluminum adjuvants are present in vaccines that protect against hepatitis B, Haemophilus influenzae type b (Hib), pneumococcus (PCV), and diphtheria, tetanus, and pertussis (DTaP). Studies, including those by Hviid, have tested this idea, and have not found a link between the aluminum in vaccines and autism.
Indeed, as we’ve explained before, childhood vaccines have been investigated for autism links based on specific hypotheses. First, the claim was that the measles, mumps and rubella vaccine might cause autism. Then, attention moved to thimerosal, a mercury-based preservative, triggering studies involving vaccines such as hepatitis B, Hib, influenza, and diphtheria, tetanus and pertussis. After that, it was aluminum adjuvants and the notion that there were too many vaccines.
Time and again, studies failed to identify an association with autism. This includes a 2013 study that covered a variety of vaccines given in the first two years of life, as it looked for a link between autism and an increasing number of proteins and sugars present in vaccines. Not every vaccine has been tested individually, but the evidence against each of these hypotheses is overwhelmingly consistent.
“Since 1998, independent researchers across seven countries have conducted more than 40 high-quality studies involving over 5.6 million people. The conclusion is clear and unambiguous: There’s no link between vaccines and autism. Anyone repeating this harmful myth is misinformed or intentionally trying to mislead parents,” Dr. Susan J. Kressly, president of the American Academy of Pediatrics, said in a statement in response to the CDC’s website change.
Studies Refute Claims on MMR Vaccines and Autism
Studies have repeatedly failed to find a link between MMR vaccines and autism.
“This is the most studied vaccine,” Mandell said. “No other potential environmental cause of autism has been studied as much as this specific vaccine.”
So much attention has been focused on the MMR vaccine because it was the subject of the initial claims of a link between autism and vaccines, perpetuated by a fraudulent study by British researcher Andrew Wakefield that was later retracted. Studies were conducted in the wake of this work to assess whether the purported connection between the vaccine and autism was real.
The new CDC webpage correctly says that reviews have found “with a high strength of evidence that there is no association with autism spectrum disorders” and MMR vaccination, but it specifies that this is based on “observational evidence only,” and not on randomized trials.
The available studies are observational out of necessity, since MMR vaccines were long ago shown to be safe and effective.
A randomized trial comparing MMR vaccination to no MMR vaccination would not be conducted anymore “because it’s unethical to purposefully withhold the vaccine,” Mandell said. “Therefore, we apply very rigorous causal methods to observational data. We don’t conduct randomized trials of parachutes either for exactly the same reason.”
“You can’t ethically not give vaccines to children knowing that diseases are out there that could cause them to suffer or die,” Offit said.
The CDC webpage also casts doubt on the existing research for various other reasons. For example, it dismisses Danish data finding no association between MMR vaccines and autism by saying the data “may be unreliable for the U.S. population.” But Hviid, who co-authored the referenced 2002 Danish study as well as a 2019 study finding no link, told us, “I see no reason why this does not generalise to the US.”
Moreover, studies on children in the U.S. and other countries around the world have also failed to substantiate a link between autism and MMR vaccination.
Evidence Contradicts Autism-Aluminum Link
The new CDC webpage tries to advance the idea that aluminum in vaccines may be linked to autism, despite research to the contrary.
The 2014 study cited as showing this link looked at data on autism cases over time in a group of children referred for services and compared it to trends in the amount of aluminum in vaccines on the CDC’s childhood schedule.
This form of study, which relies on comparing broad trends rather than looking at individual-level data, is called an ecological study. “This is the weakest form of epidemiological evidence, and we only do these kinds of studies if we have no other forms of evidence,” Mandell said.
“At best such a comparison could be hypothesis generating, and the best available evidence does not support the hypothesis,” said Hviid.
Hviid co-authored a July 15 study using individual-level records on aluminum exposure from vaccines and health outcomes in more than 1.2 million Danish children, finding no association between the vaccines and autism or other conditions.
“Our results do not support an association” between autism and the aluminum content of infant vaccines, Hviid reaffirmed in an email responding to the new CDC webpage. He added that a relationship between autism and aluminum in vaccines “is not supported by the best available evidence.”
But rather than citing this as the reassuring evidence it is, the CDC webpage goes on to cherry-pick data from two of the paper’s 15 supplementary figures and tables to cast doubt on its conclusions. This directly echoes Kennedy’s prior misuse of the study, which we have written about in detail.
One cited supplementary figure showed an increased risk of one specific autism-related diagnosis during just part of the time period studied, based on a small number of cases of that particular diagnosis. The main results of the paper, as well as other figures making other comparisons, found no such relationship.
The likelihood of finding a statistically significant result by chance alone increases the more comparisons one makes. “Secondary analyses explored over 540 comparisons, so it is expected that some are likely to be statistically significant by chance alone,” the editor-in-chief of the journal that published the Danish study wrote in an Aug. 11 response.
“You can’t just go through pages and pages and pages of tables and pick one little one out that you like and ignore all the others,” Jeffrey S. Morris, director of the division of biostatistics at the University of Pennsylvania’s Perelman School of Medicine, told us at the time.
Experts also told us that a second cited supplementary figure, on top of being cherry-picked, should not have been used to try to infer whether aluminum in vaccines caused autism in the first place because it violated statistical rules on how to properly compare groups of people.
“This is a giant fishing expedition to see if they can find anywhere where there appears to be even a slight correlation between aluminum and autism,” Mandell told us in response to the new CDC webpage claims.
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