President Donald Trump on Monday signed an executive order that could substantially alter the childhood vaccine schedule in the United States. Among other things, the order calls for the measles, mumps, and rubella vaccine to be given as three separate shots, says childhood vaccines should be spread out between medical visits as much as possible, directs healthcare officials to invest further efforts into studying the effects of the current schedule on children‘s health, and calls for more research into alternative adjuvants outside of aluminum.
It would appear that directives such as these, as they relate to the MMR vaccine, are based on what is largely a moot point, as numerous studies have put major dents in the notion that the MMR vaccine itself has anything to do with the increase in the incidence of autism.
However, the question of whether the MMR vaccine causes autism is completely different from whether receiving several different vaccines containing different bioactive ingredients influences the long-term health outcomes of recipients when those vaccines are given together versus spaced out; whether cumulative exposure to vaccine adjuvants bears any neurodevelopmental or physiological consequences in susceptible individuals — or what susceptibility even means in this context; or whether the childhood schedule as a whole has been studied with the same depth as its individual components. Evidence against an MMR-autism relationship does not automatically answer any of those questions.
Taking the latter point first, in 2013, the Institute of Medicine reviewed the safety of the childhood immunization schedule and found the available evidence reassuring. At the same time, the committee noted that few studies had been designed specifically to examine the long-term effects of the cumulative number of vaccines or other features of the schedule and recommended further comparative research. A white paper commissioned by the Centers for Disease Control and Prevention, published in 2016, came to a similar conclusion. It noted that although the available evidence supported the safety of the recommended schedule, few published studies had examined the schedule as a whole, and it laid out ways the Vaccine Safety Datalink could be used to study those questions more directly. A peer-reviewed version of the white paper likewise concluded that additional observational research comparing fully vaccinated children with children following delayed or alternative schedules was warranted. More than a decade after the IOM report, many of those broader questions — particularly those involving the schedule as a complete exposure rather than the safety of individual vaccines — have still not been studied with the same depth.
The new executive order is notable because it places several of these different questions next to one another. It calls for separate MMR vaccines in an area where the autism evidence is already extensive, while also calling for additional study of vaccine timing, sequencing, and alternatives to aluminum adjuvants — areas where the evidence is less complete. Whether three antigens given together contribute to poor neurodevelopmental outcomes is not the same question as whether cumulative exposure to an adjuvant has long-term effects.
In 2025, Dr. Niklas Worm Andersson and colleagues examined cumulative vaccine-derived aluminum adjuvant exposure in more than 1.2 million Danish children. The main finding was reassuring: Increasing aluminum exposure was not associated with a higher overall rate of neurodevelopmental disorders. A 2026 systematic review of 59 human studies was similarly reassuring overall, concluding that the available evidence did not support a causal relationship between aluminum-adjuvanted vaccines and serious or long-term health outcomes. But the number 59 requires some qualification when it comes to autism specifically. Only three of the studies included in the review examined autism: two ecological studies that the reviewers judged to be at critical risk of bias, and the large Danish cohort described above. In that sense, the review’s reassuring conclusion regarding autism depended heavily on the Andersson study rather than on dozens of independent studies examining aluminum exposure and autism.
But the Andersson study also had limitations that make it difficult to treat it as the final word on every aluminum question. Aluminum exposure varied partly because vaccine formulations and recommendations changed over time, meaning cumulative dose was closely related to birth cohort. Some earlier-born children could not have reached the highest exposure categories available to later-born children, while autism recognition and diagnosis were also changing over the same period. The study’s supplementary material was also corrected shortly after publication. In the corrected supplementary analyses, among children born between 2007 and 2018 — a period in which variation in aluminum exposure remained — each additional 1 mg of vaccine-derived aluminum was associated with a higher rate of Asperger’s syndrome.
That signal was not present in the study’s primary full-cohort analysis and should not be treated as evidence of causation, but neither is it simply explained by comparing the earliest birth cohorts with the latest ones. Importantly, this study, too, does not answer whether the timing of aluminum exposure matters, whether some children process or retain injected aluminum differently than others, or whether findings from one national vaccine schedule can be applied without qualification to materially different schedules. Those are entirely separate questions.
Such are the limitations in the current evidence base. Evidence largely disqualifying MMR as a cause of autism is often extrapolated to the vaccine schedule as a whole, even though, as the IOM and CDC have themselves acknowledged, broader questions about the schedule have not been studied with the same depth. At the same time, questions surrounding individual vaccines like MMR have been studied extensively and, in many respects, answered.
TRUMP SIGNS ORDER SHRINKING CHILDHOOD VACCINATION SCHEDULE
The executive order now creates an opportunity to study these questions for what they are: different questions. The value of that research will depend, in part, on whether its findings are allowed to answer what was actually studied rather than being stretched to settle questions that were not.
The problem may therefore be less a lack of information than an inability on both sides of the debate to tolerate distinctions in the evidence — to acknowledge where the science is strong, where it is weaker, and where uncertainty still remains.
Yasin Muhammad is a biomedical researcher and cell biologist.