Let me make a small concession on behalf of the medical community: The CDC is technically correct when it asserts, as it did this week in a surprise update to its website, that “studies have not ruled out the possibility that infant vaccines cause autism.” But the underlying logic of this change clearly goes beyond the wispy double negative. Robert F. Kennedy Jr. has already said that he believes in the affirmative: Vaccines do cause autism. And because he is now secretary of Health and Human Services, he can order his bureaucracy to lean ever further toward that same belief. A causal link hasn’t not been found, the CDC is saying now—at least not completely, not quite yet.
If this pretzel logic is confusing, that’s the point. Bewilderment and doubt are among the anti-vaccine movement’s most powerful weapons. It’s true that doctors cannot say with absolute certainty that some ingredient in some vaccine, or combination of vaccines, does not contribute in some way, however small or large, to the rise in autism diagnoses. We also can’t rule out the possibility that infant vaccines cause tornadoes or bad movies. Uncertainty is inseparable from science.
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Kennedy has firsthand knowledge of how difficult it is to prove a medical assertion. He began his crusade against immunizations 20 years ago, with the argument that the vaccine preservative thimerosal was causing a spike in autism rates. In his discredited 2005 article on the topic, he said he was “convinced that the link between thimerosal and the epidemic of childhood neurological disorders is real.” In fact, the evidence was very weak, and additional real-world observations have further undermined his claim. Throughout the 1990s and 2000s, thimerosal was removed, as a precaution, from childhood inoculations in many developed countries, and yet autism diagnoses continued to climb. Denmark took the preservative out of its shots in 1992, for instance, yet experienced a fivefold increase in autism diagnoses among young children by the end of the millennium.
In a New York Times interview yesterday, Kennedy acknowledged that studies had shown no link between thimerosal and autism. But he and his fellow anti-vaccine activists are undeterred by this contradiction. They have expressed little, if any, regret about their misguided crusade. (In fact, Kennedy has lately taken his anti-thimerosal campaign global.) But even if they were to grant that this ingredient is not, in fact, a cause of autism, they’d still be pointing at all the other vaccine components. Can each and every one be ruled out as a risk? Notably, the word thimerosal does not appear in this week’s update from the CDC. (A separate, preexisting page on CDC.gov says that the preservative has “no connection with autism.”) Instead the agency now points to another boogeyman ingredient: aluminum. That’s what warrants “further investigation,” circa 2025. (HHS Press Secretary Emily Hilliard told me that Kennedy supports the removal of thimerosal from all U.S. influenza vaccines, and that his “comprehensive review of autism’s causes” will emphasize “transparency, reproducibility, and gold-standard science.”)
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Kennedy has a clever way of playing with the evidence: He will hack apart robust results that support the safety of vaccines while canonizing any bit of information that could be seen to go the other way. The CDC-website update is a perfect specimen of this disordered scientific thinking. It shows how drops of doubt can be squeezed from even the most rock-solid data. One of the site’s citations, for example, is to a major study of aluminum-based vaccines that came out in the Annals of Internal Medicine earlier this year. Danish researchers examined the medical records of more than 1 million children and found no association between the amount of aluminum present in vaccines administered early in life and 50 different medical conditions. When that analysis was published, Kennedy demanded that it be retracted: “The only thing this study proves is the thorough corruption of the scientific journals that publish such garbage-in, garbage-out exercises in statistical manipulation,” he wrote in an op-ed last August. Yet now, improbably, the CDC is pointing to this very study in support of its double-negative conclusion. Are aluminum vaccines a cause of autism? Not necessarily no.
Echoing Kennedy’s assertions, the CDC site maintains that one should just ignore the Danish study’s overall results in favor of its secret truth. The paper’s real result, it suggests, is hidden in the supplementary tables, where one reveals that “moderate” aluminum exposure is linked to higher rates of neurodevelopmental conditions. But on closer look, the table also shows that any such relationship disappears at higher doses of aluminum—a quirky finding that should make the whole idea appear unsound.
The CDC continues on to cite aluminum’s apparent link to reported diagnoses of Asperger’s syndrome. This conclusion, too, is highly suspect: The data point in question, buried in supplemental figure 4, was seen only in a tiny subsample of 51 children with the condition. A more complete analysis of 3,000 children with neurologic disorders in the study found something like the opposite: Greater levels of aluminum were associated with fewer problems. None of this should be treated as a demonstration that aluminum-based vaccines are dangerous or beneficial; the details of these supplemental tables show only that science is a messy business. (Anders Hviid, the senior author of the paper, has responded to Kennedy’s assertions by maintaining that his study “does not provide support for the hypothesis that aluminum used as adjuvants in vaccines are associated with increased risks of early childhood health conditions.”)
The best doctors are aware of all these opportunities for confusion. Well-calibrated doubt is an important tool in medicine—more essential than a stethoscope or an X-ray—and the uncertainties it yields should be acknowledged and communicated. So why do physicians keep insisting that vaccines don’t cause autism? They are not “lying to you,” as Kennedy has alleged. Instead, they are doing what doctors are supposed to do: digesting a large amount of information in order to produce the most reasonable conclusion. They know that double negatives don’t help in matters of life and death. (As a pathologist, I try not to tell patients, “Well, you don’t not have cancer.”) Professional skeptics like Kennedy thrive on raising questions—but the public thrives on getting answers.