On Sept. 18, the vaccine advisory committee for the Centers for Disease Control and Prevention voted to change its recommendation on the combined vaccine for chickenpox and measles, mumps and rubella, called the MMRV vaccine. Cases of these diseases in the U.S. plummeted after the introduction of vaccines decades ago.
If adopted by the CDC, the new guidance would remove the combined MMRV vaccine as a recommended option for children under the age of 4, who typically receive a first dose of the MMR and chickenpox vaccines at 12 to 15 months of age. The recommendation would not alter access to the MMR and chickenpox vaccines when given as two different injections on the same day. The MMRV vaccine would remain the preferred option for kids getting their second (and final) dose, recommended between ages 4 and 6.
Giving MMR and chickenpox vaccines separately is already the more common practice for kids getting their first dose of these vaccines, due to a slightly better side effect profile for the separate shots. When given as a first dose at the typical age, the combined shot comes with a slightly elevated rate of febrile seizures — or seizures that accompany a fever. These are generally considered benign in the long term but can be distressing and can lead to emergency room visits.
Because of this, CDC’s longstanding recommendation has been to prefer giving the shots separately for the first doses, but to give parents the option of choosing the combined MMRV shot if they prefer it after discussing the benefits and risks with their doctors. The new recommendation reduces this flexibility.
“You just basically took that choice away from the parent,” Dr. Paul A. Offit, a vaccine expert at the Children’s Hospital of Philadelphia, told us.
Vaccine experts questioned why the panel was revisiting the issue at all. “We have not seen an ongoing challenge with this since the original decision was made to put this in the hands of the practitioner and the parent, and with full data, full disclosure of what the information is,” Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, told us. “This was a solution looking for a problem,” he added.
We spoke to experts about claims on the MMRV vaccine made at the Advisory Committee on Immunization Practices meeting as well as the implications of the vote.
What are the side effect concerns with MMRV?
The reason the CDC has long preferred giving the chickenpox vaccine separately when administering the first dose of the MMR vaccine is that the MMRV vaccine comes with a slightly elevated risk of febrile seizures, compared with getting two different shots separately at the same visit.
The likelihood of having a febrile seizure from any cause peaks between 14 and 18 months of age, according to a CDC presentation given at the meeting. Around 2% to 4% of young children have experienced at least one febrile seizure, often while sick with infectious diseases. Vaccine side effects also can include fevers, which can, in a small percentage of cases, lead to febrile seizures.
But febrile seizures are rarely associated with long-term problems. “Every pediatrician is experienced in febrile seizures, and we know that the prognosis is excellent,” Dr. H. Cody Meissner, a member of the Advisory Committee on Immunization Practices and a professor of pediatrics at Dartmouth Geisel School of Medicine, accurately said during the meeting. Febrile seizures can be anxiety-provoking for family members, he said, “and that’s entirely understandable,” but they are “not associated with any sort of impaired performance or neurocognitive development or school problems.”
Meissner is listed as having been on ACIP in 2009, when the most recent prior MMRV recommendations were made, and he was a member of the work group at the time that looked into the MMRV issue.
The combination MMRV vaccine in question, made by Merck, was approved by the Food and Drug Administration in 2005, and ACIP initially recommended it as the preferred option for both doses. This decision was in keeping with a general preference for combination vaccines, allowing vaccination with fewer shots.
Vaccine safety monitoring using the CDC’s Vaccine Safety Datalink system, as well as data from a Merck study, subsequently showed that giving the MMRV vaccine for the first dose was associated with a higher rate of febrile seizures than giving the MMR and chickenpox vaccines as two separate shots at the same time.
Using the combined MMRV vaccine in 1-year-olds, rather than the two-shot approach, leads to one extra febrile seizure per 2,300 to 2,600 children, according to a CDC background document posted before the ACIP meeting. There is no such increase observed with the second dose in older children.
After reviewing new data on febrile seizures in 2008, ACIP voted to remove the preference for MMRV over the separate shots. The MMRV had been unavailable at the time in the U.S. anyway, due to manufacturing constraints, the CDC materials said.
The following year, in 2009, the committee decided to recommend either the two-shot approach or the combined MMRV vaccine for children under age 4 getting a first dose, while giving guidance that any provider considering giving MMRV should discuss the benefits and risks with the parents. In implementing the recommendations, the CDC said that the two-shot approach should be preferred, in the absence of a parental or caregiver preference. For the second dose, typically given between ages 4 and 6 when children are less susceptible to febrile seizures, the committee recommended the combined MMRV shot as the preferred option.
“If a parent specifically requests fewer injections and understands the small additional risk associated with [MMRV], the current guidelines allow them to make that choice,” Dr. Carlos O’Bryan Becerra, a family physician in Ventura, California, and a former vaccine science fellow for the American Academy of Family Physicians, told us via email, referring to the longstanding CDC guidelines since 2009.
Children end up with equivalent protection from measles, mumps, rubella and chickenpox, according to the CDC briefing document, no matter their parents’ choice.
What claims were made about MMRV vaccine testing and safety?
During the meeting, some ACIP members pursued misleading lines of questioning on MMRV safety.
Retsef Levi, a member of ACIP and a professor of operations management at the Massachusetts Institute of Technology’s business school, speculated without evidence that febrile seizures after vaccination may be different than febrile seizures from infection. “I think we need to investigate that, because if it’s not, potentially our assumptions about the long-term and the short-term implications may not be true,” he said.
Meissner replied to Levi during the meeting that febrile seizures are in fact well defined and questioned “putting limited resources” into further study. “The vast majority of febrile seizures do not occur in association with vaccines,” he said.
The “current scientific and medical consensus” is that febrile seizures after vaccination “are clinically indistinguishable from those triggered by fever from other causes, such as a viral illness like the measles or varicella viruses the vaccines prevent,” Dr. James Bigham, a family physician in Madison, Wisconsin, and a former AAFP vaccine science fellow, told us via email.
ACIP member Vicky Pebsworth, a nurse with a doctorate in public health who has ties to anti-vaccine groups, also suggested that febrile seizures could lead to long-term problems, including ADHD and cognitive decline, citing a 2023 review article.
Bigham said that the 2023 article “does not represent current medical consensus” that the “vast majority of febrile seizures are benign and do not cause brain damage, ADHD, cognitive decline, or epilepsy.” He said that a less common type of febrile seizure, called complex febrile seizure, or febrile seizures in children with preexisting neurological conditions or a family history of epilepsy, may be associated with long-term risks.
Dr. Jonathan Temte, a professor of family medicine and community health at the University of Wisconsin School of Medicine and Public Health, emailed us a link to an analysis suggesting that any long-term issues are likely caused by the underlying disorder, rather than the febrile seizure itself. Temte led the ACIP MMRV Safety Work Group, which met leading up to the ACIP recommendations in 2009.
Dr. Evelyn Griffin, an obstetrician-gynecologist from Louisiana who was added to ACIP this month, asked whether the MMRV vaccine had been tested in a trial with a saline placebo. She suggested that such a study “is probably the most ethical thing to do in a day and age where parents are, through full, informed dissent, not getting their children vaccinated.”
To be clear, most parents in the U.S. do continue to get their children vaccinated, although vaccine coverage for kindergartners has fallen.
MMRV was tested in randomized clinical trials that looked at children’s immune responses to the combination vaccine versus the two-shot strategy, finding it to be equivalent. Trials also assessed safety, finding an elevated rate of fever after MMRV, which motivated the later monitoring for febrile seizures.
A CDC expert attending the ACIP meeting replied that families participating in the trial would need to consent to receiving either the vaccine or the placebo. “I think at a time where we’re seeing the highest measles cases we’ve seen in many years, it would be concerning to ask parents to risk the chance that they would receive a vaccine that would not end up protecting them from measles, mumps, rubella or [chickenpox] disease,” the expert said, referring to the fact that some kids would get placebo vaccines.
President Donald Trump also misled about the MMR and MMRV vaccines in a Sept. 22 press conference about autism. “The MMR, I think should be taken separately. This is based on what I feel. The mumps, measles … the three should be taken separately,” he said, stating that “there could be a problem” with mixing them. “The chickenpox is already separate, because when that got mixed in — I guess they made it four for a while — it really was bad,” he said.
The benefit of separating the MMR and chickenpox vaccines into two shots is a small reduction in the rate of febrile seizures in young children, as we’ve said. There is no evidence linking either the MMR or MMRV vaccines to autism.
What was unusual about this ACIP recommendation?
ACIP did not follow its normal process for considering the available evidence before making new recommendations. This involves not only looking at the public health problem being addressed and the evidence of benefits and harms of vaccination, but also issues such as the feasibility of implementing the new recommendation, whether it is cost-effective and how it might affect different groups of people. Ordinarily, this process begins with carefully gathering the available evidence and systematically assessing its certainty.
“Are we going to have a thoroughly vetted, evidence-to-recommend framework presentation that looks at all the harms, benefits, acceptability, feasibility, with input from practicing clinicians and liaisons, in order to make an informed decision?” Dr. Jason Goldman, president of the American College of Physicians and a liaison to the ACIP committee, asked during the meeting.
Dr. Demetre Daskalakis, former director of the CDC’s National Center for Immunization and Respiratory Diseases, told us that the timing of when the agenda was set for the meeting allowed insufficient time for the normal process of evidence review before making a recommendation. “There is nothing emergent about the MMRV,” he said. “There’s no new data.”
Daskalakis resigned from his position in August, citing an inability to “serve in an environment that treats CDC as a tool to generate policies and materials that do not reflect scientific reality and are designed to hurt rather than to improve the public’s health.”
Daskalakis said that there may be issues in implementation “that no one has thought about” because the process was not completed in a systematic way. “The Secretary dictated the schedule, this agenda,” he said, leaving a month for preparation. “Setting the agenda with no lead time is an aggressive act,” he said.
Ordinarily, there would be a year in lead time, he said. The relevant work group to review the vaccine schedule “has not actually met,” he said, and so could not discuss the topic.
ACIP formally adopted its framework for evidence-based recommendations in 2010. Temte, the leader of the MMRV Safety Work Group that reviewed the evidence before ACIP’s 2009 recommendation, described a lengthy deliberative process, also outlined in CDC materials.
Temte said that his work group met “2-4 times per month, over most of a year to carefully review the evidence.” This included reviewing two independent studies that were done after the vaccine was approved and “basic issues of febrile seizure bolstered by multiple consultants,” he said. The researchers also held focus groups to assess parents’ attitudes, he said. CDC materials also describe consideration of provider attitudes and how the recommendations would be implemented.
We asked the Department of Health and Human Services for comment on Daskalakis’ account of how the ACIP agenda was set and why the evidence-to-recommendation framework could not be followed.
“Susan verbally approved the ACIP agenda and the date of the meeting scheduled for the week on August 4th,” an HHS spokesperson told us via email, referring to former CDC Director Susan Monarez, a microbiologist and immunologist. “ACIP is guided by gold standard science and will make any recommendations based on the totality of evidence presented to them. HHS has not, and will not, limit access to vaccines.”
Monarez testified Sept. 17 that she had been fired from her position after serving for 29 days after she refused to commit in advance to approving all ACIP recommendations. Monarez also said that HHS Secretary Robert F. Kennedy Jr. told her “the childhood vaccine schedule would be changing starting in September and I needed to be on board with it.” An HHS spokesperson told us via email that the CDC’s acting director now “reviews and approves those recommendations.” Kennedy has said he did not ask Monarez to pre-approve ACIP recommendations and that he fired her because she told him she was not “trustworthy.”
The current CDC acting director is Jim O’Neill, an advisor to Kennedy who served in HHS under President George W. Bush and has a background in venture capital.
The American Academy of Pediatrics did not send a representative to the meeting and has come out with its own vaccine recommendations, which maintain MMRV as an option for younger children.
How will this decision affect the vaccination rate?
It’s unknown how the change would affect the rate of vaccination against MMR and chickenpox, experts told us. CDC data presented at the advisory meeting stated that 15% of kids getting vaccinated get MMRV for their first dose, rather than the two-shot approach. In different states, that percentage ranges from 5% to nearly 32%.
“That will be an experiment,” Dr. William Schaffner, a professor of infectious diseases at the Vanderbilt University Medical Center, told us. If a parent desires fewer total shots for the first dose, he said, the pediatrician or family doctor will have to say, “We have to give two vaccines instead of just the one, in addition to all the other vaccines.” And if a parent chooses to then delay a vaccine, there is a risk the child will not go on to get it later.
Osterholm agreed that it’s unknown how the decision will affect vaccine uptake.
Even a small drop in vaccination rate can be consequential. “Recent CDC surveillance data reveal a troubling decline in kindergarten vaccination coverage, where rates for measles, mumps and rubella — potentially serious and fatal diseases — have fallen below 95% threshold, which is required to sustain measles herd immunity,” Dr. Rick Haupt, head of infectious disease and vaccines, medical and scientific affairs at Merck, said during the meeting. “Considering these trends, any policy decision that compromises the clarity or consistency of vaccination guidance for MMRV has the potential to further diminish public confidence.”
How does this change which vaccines children can get?
The recommendation, if adopted, has ramifications for whether parents can feasibly choose the combined MMRV vaccine for their child’s first dose. As we’ve said, children will still be able to get separate MMR and chickenpox vaccines.
“Several ACIP members speak frequently during the committee’s meetings about prioritizing parental choice in vaccination decisions,” Jason Schwartz, a professor at the Yale School of Public Health with expertise in vaccine policy, told us via email. “Yet this change to its MMRV vaccine recommendations adds new barriers—in the form of eliminated or uncertain financial coverage—for parents of children in the affected age group who prefer this option for their child.”
For those children on private insurance, coverage would become more uncertain but would be “likely preserved for most,” Schwartz told us. The recommendation would mean that these insurers are no longer required to cover the MMRV vaccines at no cost. But a large association of health insurers announced that it would continue to cover vaccines that had been recommended by ACIP as of Sept. 1 through the end of 2026.
The decision effectively removes MMRV as an option for some other children. About half of all children in the U.S. are eligible for vaccines through the Vaccines for Children program, which provides vaccines at no cost to children who meet certain criteria, including being uninsured or underinsured, eligible for Medicaid, or an American Indian or Alaska Native. The updated vote established that the MMRV vaccines will no longer be provided for free via this program for kids under 4, Schwartz confirmed.
Some other children get vaccines via the Children’s Health Insurance Program, which provides coverage for kids who are not eligible for Medicaid. This group also will no longer be eligible for free MMRV vaccines under age 4, Schwartz said.
“By removing the option of using the [MMRV] combination vaccine, ACIP is effectively limiting parental choice,” family physician O’Bryan Becerra said.
Jessica McDonald contributed to this story.
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