CDC panel votes to separate MMR and varicella vaccines for children under four

In a landmark vote, the CDC's Advisory Committee on Immunization Practices (ACIP) approved a major change in how three childhood vaccines are administered. On Thursday, the 12-member panel voted 8-3, with one abstention, that children aged four and under should no longer be offered the combined measles-mumps-rubella-varicella (MMRV) vaccine. Instead, the panel recommended that the measles, mumps and rubella (MMR) and varicella, or chickenpox, shots be given separately. The decision does not withdraw approval for either vaccine; both remain on the recommended schedule. The vote marked the first since Robert F. Kennedy Jr. took the helm of the panel, which includes members who have advocated against vaccine use. The panel debated the change at the CDC’s headquarters in Atlanta, Georgia.

Merck & Co., which manufactures the MMRV combination, criticized the decision, saying it had occurred in the absence of new scientific data and in contrast to years of evidence affirming the current immunization schedule. Experts voiced concerns that the shift could impede access to protection against diseases that historically sickened large numbers of people. Dr. Aaron Milstone, pediatric infection control chief at Johns Hopkins Children’s Center, warned that the change could reduce access to protection against measles, mumps, rubella and varicella.

The panel noted that the recommendation still requires approval from the acting CDC director, Jim O’Neill, who recently replaced Dr. Susan Monarez after she said she was fired for defending scientific integrity. The panel is reconvening, and further guidance is expected on other vaccines, including the hepatitis B schedule.

Under the current practice, first doses of MMR and varicella are often given separately on the same visit unless a parent requests the combined MMRV shot. Data presented to ACIP show that when first doses are given separately on the same visit, about 15 out of every 100 children develop a fever and four out of every 10,000 suffer a febrile seizure. When the shots are given as a combined first dose, the risk of fever rises to 22 out of every 100 children, and the risk of a febrile seizure doubles to eight out of every 10,000 children. Health care providers emphasize that a febrile seizure, while dramatic in appearance, typically lasts only a few minutes and rarely leads to long-term complications. The second dose of MMRV, given after age four, has not shown the same increased risk, which means the new policy would not affect its use as a second dose.

The four vaccines under discussion are designed to protect against diseases that once caused widespread illness and death. Measles infections were historically tens of thousands per year in the United States before vaccines; estimates place past measles infections around 3 to 4 million annually, with 48,000 hospitalizations and 400 to 500 deaths. Chickenpox infections were even more common, with about 4 million children infected yearly and between 10,500 and 13,500 hospitalizations and 100 to 150 deaths. The United States has held measles elimination status since 2000, though the current year has seen 1,491 cases, the most since elimination was declared. Chickenpox continues to affect an estimated 100,000 to 150,000 individuals annually in the United States.

If approved, the new approach would align U.S. guidelines with countries such as Australia and France, which require separate MMR and varicella vaccines for the first dose. Meanwhile, Canada and Italy offer parents a choice between the MMRV and separate MMR and varicella shots for the first dose. The United Kingdom, by contrast, plans to begin offering the combined MMRV shot as a first dose in January 2026.

The MMRV vaccine was first approved in 2005 and combines the two-dose MMR schedule (12 to 15 months and 4 to 6 years) with a two-dose varicella schedule at the same ages. The current policy avoids the combined shot as a first dose for children under four due to the elevated fever and febrile seizure risk, but continues to permit its use as a second dose.

Healthcare professionals and public health experts stress that the overall level of protection remains consistent across both approaches. The debate now centers on access, parental preferences, and the practical implications of administering vaccines on separate visits. The timeline ahead hinges on final sign-off from the CDC director and potential guidance updates tied to broader immunization scheduling reforms.