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As anti-vaccine sentiment rises nationally, prominent people have proposed splitting combination vaccines into individual shots. For example, the new acting director of the Centers for Disease Control and Prevention (CDC) recently said that manufacturers should separate the combined measles-mumps-rubella (MMR) vaccine into three different (“mono-valent”) vaccines.
While this idea may sound new and aligned with parental choice, it risks undermining childhood immunization rates and increasing preventable illness and death.
How the Idea of Splitting Vaccines Became Popular
After the success of the polio vaccine campaign in the 1950s, vaccine researchers developed individual vaccines from 1963-1969 against three viruses—measles, mumps, and rubella—that were associated with birth defects, infertility, deafness, and death.
In 1971, a combined vaccine (MMR) was introduced, and vaccine uptake in children increased dramatically. Parents preferred a combination vaccine, because it meant fewer injections and fewer clinic visits, and pediatricians preferred it because it simplified vaccine ordering and storage. Illness from these three viruses fell dramatically.
The campaign to separate the MMR vaccine into three separate vaccines began in the late 1990s after a fraudulent study was published claiming a link between MMR vaccination and autism. At a press conference announcing this study, British physician Andrew Wakefield called for the vaccines against these three viruses to be split and administered separately.
Wakefield’s claims were based on fraudulent data, his paper was withdrawn, and his medical license was eventually revoked. Nevertheless, his message resonated with parents in the United Kingdom, and many began rejecting vaccination. In the United Kingdom, between 1998 and 2008, measles cases rose more than tenfold, leading to tens of thousands of preventable infections and dozens of deaths.
That dangerous idea has now been revived in 2025 in the United States.
The Combination MMR Vaccine is Safe, Effective, and Practical
The MMR vaccine has one of the strongest safety records of any medical intervention. More than half a century of research, across hundreds of millions of doses, has confirmed its effectiveness and safety.
The notion that combining vaccines “overwhelms” the immune system is not correct. The human immune system is designed to recognize and fight off thousands of antigens daily from microbes that we encounter from physical contact, breathing, eating, and drinking. The three antigens in the MMR vaccine represent an almost trivial addition to that constant exposure.
Moreover, studies repeatedly show that immune responses are equivalent whether the vaccines are given together or apart. The MMR vaccine elicits durable protection against all three infections without increasing the risk of side effects. Immunologically, there is no evidence that separating the vaccines improves the body’s response or reduces adverse reactions.
At a population level, combination vaccines simplify logistics for families and health systems. Each additional shot adds cost, waste, and the possibility of human error.
Why Splitting Combination Vaccines Will Harm Health
Producing single-dose versions of measles, mumps, and rubella vaccines in the United States would be nearly impossible. None have been manufactured for decades. Restarting production would require new facilities, clinical trials, and regulatory approval, an enormous cost for a product that offers no improvement in safety or efficacy.
Studying such vaccines would also be unethical. To test separate doses, researchers would need to delay or withhold protection for large numbers of children. That would expose them to unnecessary risk for infections we already know how to prevent.
Even if separate vaccines were produced, vaccination coverage would likely drop. Each additional appointment needed creates another opportunity for a child to miss the opportunity to be protected against preventable diseases.
The persistence of this idea has a lot to do with psychology and almost nothing to do with science. For some parents, splitting vaccines seems to restore control over a complex system they do not fully trust. The impulse is understandable, but it’s misplaced. The MMR vaccine was designed precisely to reduce complexity and benefit children.
The Challenges to Vaccine Confidence From Splitting Vaccines
I do not believe that vaccine confidence will be restored by more research studies. The data are already abundantly clear that the MMR vaccine is safe and effective.
Rather, we need trusted people at all levels to acknowledge parents’ fear and uncertainty about their children’s health, as well as acknowledge that our healthcare system rarely feels caring or focused on health—or, indeed, much like a system at all. We also need those trusted people to repeat the evidence, rather than the misinformation.
The MMR debate demonstrates how fragile that trust remains, and how easily it can be eroded when prominent people promote ideas that are not grounded in science. I worry that revisiting this debate diverts attention from the most urgent priorities we have for infectious diseases: improving disease surveillance, strengthening immunization programs, and expanding coverage for vaccines that remain underused, such as HPV. What’s more, each time we must defend scientific consensus against misinformation, we waste scarce public health resources and, ultimately, put our collective health at risk.
