CDC Looking to Break Up MMR Vaccine?

On Oct. 6, 2025, the acting director of the U.S. Centers for Disease Control and Prevention (CDC), Jim O’Neill, called on the manufacturers of the live attenuated MMR (measles, mumps, rubella) vaccine to break up the biologic and develop separate shots for the three diseases it targets. “I call on vaccine manufacturers to develop safe monovalent vaccines to replace the combined MMR and “break up the MMR shot into three totally separate shots,” wrote O’Neill in a post on X.^{1 2 3}

The comments by O’Neill, who also serves as deputy secretary of the U.S. Department of Health and Human Services (DHHS), signal a possible rethinking by the U.S. government on the matter of the safety, effectiveness and necessity of using combination vaccines like the MMR. Within the pharmaceutical, medical trade, and public health industries, combining vaccines is seen as the future of vaccination programs because of their potentially lower overall costs and greater convenience, allowing for fewer injections while adding on an ever-expanding number of vaccines to the federally recommended child and adult vaccination schedules. Combination vaccines are viewed by these industries as more efficient because they save administration time and increase compliance and coverage with multiple vaccines given all at once.^{4 5 6}

Combination Vaccines Key to Growing the Market

In an article published earlier this year on the Gavi, the Vaccine Alliance website, Linda Geddes wrote:

[E]xperts are eyeing combination vaccines as a solution to another pressing problem: although new vaccines are urgently needed, the logistics of delivering them is increasingly hindered by their own success. … Combining some of these new vaccines into existing shots might simplify their delivery. It could also potentially reduce their cost.⁶

According to Bill Hausdorff, PhD, head of PATH’s Center for Vaccine Innovation and Access in Washington, DC:

Despite the technological beauty of all these potential and current vaccines, we’ve hit a crossroads where the ability to make new vaccines is well surpassing countries’ abilities to absorb them. They don’t have the money; they don’t have room in their immunization schedules.⁶

“If you have a single vial, you have less equipment, fewer issues around delivery and disposal of needles. You’re also reducing the environmental impact of vaccination,” head of policy at Gavi Marta Tufet Bayona, PhD said.⁶

The American Academy of Pediatrics (AAP), along with the American Academy of Family Physicians, (AAFP) and the Advisory Committee on Immunization Practices (ACIP), recommend the use of combination vaccines. Of the 112 vaccines licensed by the U.S. Food and Drug Administration (FDA), 17 of them are combination vaccines.^{4 7}

A study published in the journal Vaccine (Basel) in March 2025 stated that the use of combination vaccines “can be regarded as a substantial modernization in immunization.” The authors stated, “Administration of multiple antigens simultaneously provides a simplified strategy for enhancing vaccination rates. It may ease vaccination schedules and ensure patient compliance.”⁷ They added:

Combination strategies will be crucial in optimizing immunization programs as new vaccines emerge, with minimal logistical burdens. … [T]he quest for developing combination vaccines through ongoing research and development is crucial for improving global health.⁷

It is unclear how serious is acting CDC director O’Neill’s notice to Merck & Co. and GSK (GlaxoSmithKline)—the two makers of MMR vaccines (M-M-R-II and Priorix) licensed in the United States—to break up their MMR vaccines into individual monovalent shots. O’Neill’s post did not offer details on why or how the move should be implemented.

Questions About MMR Safety Testing Remain

Of course, one answer to the “why” is the claim that the MMR vaccine was never adequately tested for safety. As Amber Baker wrote in an article in The Vaccine Reaction last month, although combination vaccines have been administered for decades and are considered safe “based on pre-licensure clinical trial data and post-marketing surveillance,” much of the evidence for the safety of a number of currently recommended childhood vaccines came from “studies that did not use true inert placebos as a control for determining the safety and effectiveness of new vaccine products.”⁸

“Historically, clinical trials of new childhood vaccines have compared experimental products not with saline or another inactive substance, but with a ‘bioactive placebo,’ such as another licensed vaccine or a vaccine ingredient like an adjuvant,” wrote Baker. “These active comparators carry their own risks, making it difficult to definitively establish whether co-administration is truly risk-free.”⁸

FDA documents from 1978 regarding the pre-licensure safety trials of Merck’s M-M-R-II, for example, highlight some of the major weaknesses with them. The documents indicate that the M-M-R-II product was licensed based on a trial with a total of just 834 children and with no control group. In the trial, the vaccine was reviewed for safety for only 42 days, during which one-third of the children developed gastrointestinal and respiratory issues.”^{9 10}

It should be noted that O’Neill’s idea of returning to separate monovalent measles, mumps, and rubella vaccines so parents can have a choice of which shots to give to their children and when to give them is not so radical as some may think. After all, it wasn’t that long ago that stand-alone vaccines for each of those childhood diseases were still available in the U.S. On Oct, 21, 2009, Merck simply decided to discontinue the production of separate measles, mumps and rubella vaccines.^{11 12}

In an article written by pediatricians and published in the December 2009 issue of AAP News, it was stated:

Mark Feinberg, MD, PhD, vice president of medical affairs in Merck’s division of vaccines and infectious diseases, indicated on Oct. 21 [2009] that his company had heard from numerous organizations, including the AAP Committee on Infectious Diseases (COID), requesting that these monovalent vaccines not be reintroduced. Monovalent measles, mumps and rubella vaccines are not needed. Their use would unnecessarily delay administration of the three vaccine components, leaving children susceptible for a longer period of time to serious, life-threatening diseases. Use of monovalent vaccines also would increase the number of injections a child would need and increase the number of visits required to complete the vaccination series.¹³

Merck’s decision to discontinue production of separate measles, mumps and rubella vaccines in 2009, when Merck had long been the sole supplier of MMR vaccines in the U.S., appears to have been primarily influenced by the AAP and the CDC’s ACIP, which took the position stating a “general preference for combination vaccines.”^{11 12}

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