A key committee of the U.S. Centers for Disease Control and Prevention voted Thursday to alter its recommendation on an early childhood vaccine, after a discussion that at times pitted vaccine skeptics against the CDC’s own data.

After an 8-3 vote with one abstention, the CDC’s Advisory Committee on Immunization Practices will no longer recommend that children under the age of 4 receive a single-shot vaccine for mumps, measles, rubella and varicella (better known as chicken pox).

Instead, the CDC will recommend that children ages 12 to 15 months receive two separate shots at the same time: one for mumps, measles and rubella, or MMR, and one for varicella.

On Friday morning, the group decided unanimously to table an anticipated vote on changes to the hepatitis B vaccination schedule, after vaccine skeptics installed on the committee raised concerns that a proposal to delay the first dose by a month didn’t go far enough.

ACIP member Vicky Pebsworth, a nurse who serves as research director for the National Vaccine Information Center, an organization long criticized for promoting inaccurate vaccine information, challenged the previous day’s presentation by CDC staff on the vaccine’s safety.

She criticized the CDC for glossing over side effects such as fever, sleepiness and fussiness.

“These are not trivial reactions,” Pebsworth said. “I personally think we should be erring on the side of caution and adopt a more prudent vaccination policy.”

The group is slated to vote later Friday on changes to the COVID-19 vaccine.

The MMRV vote represents a relatively small change to current immunization practices. But doctors said the lack of expertise and vaccine skepticism on display during much of the discussion would only further dilute public trust in science and public health guidance.

“I think the primary goal of this meeting has already happened, and that was to sow distrust and instill fear among parents and families,” Dr. Sean O’Leary, chair of American Academy of Pediatrics’ Committee on Infectious Diseases, said Thursday during a news conference over Zoom.

“What we saw today at the meeting was really not a good-faith effort to craft immunization policy in the best interest of Americans. It was, frankly, an alarming attempt to undermine one of the most successful public health systems in the world,” O’Leary said. “This idea that our current vaccine policies are broken or need a radical overhaul is simply false.”

Giving the MMR and chickenpox vaccines in the same shot has been associated with a higher relative risk of brief seizures from high fevers in the days after vaccination for children under 4 — 8 in 10,000 children typically have febrile seizures after receiving the combination shot, compared with 4 in 10,000 who receive separate MMR and chickenpox shots at the same time.

Distressing as they are for family members to witness, seizures are a relatively common side effect for high fevers in young children and have not been associated with any long-term consequences, said Dr. Cody Meissner, a former pediatric infectious diseases chief at Tufts-New England Medical Center who is serving on ACIP for the second time (he previously served under Presidents George W. Bush and Obama).

The problem with splitting vaccines into multiple shots is that it typically leads to lower vaccine compliance, Meissner said. And the risks of not vaccinating are real.

“We are looking at a risk-benefit of febrile seizures … as compared to falling below a 95% coverage rate for herd immunity, and the consequences of that are devastating, with pregnant women losing their babies, newborns dying and having congenital rubella syndromes,” said Dr. Joseph Hibbeln, a psychiatrist and neuroscientist and another current ACIP member.

Meissner, Hibbeln and Hilary Blackburn were the only three members to vote against the change.

The first day of the meeting ended with a vote regarding continued coverage of the MMRV shot under the CDC’s Vaccines for Children Program, a publicly funded service that provides immunizations to nearly half of the nation’s children. The program currently only covers shots that ACIP recommends.

As chair Martin Kulldorff called the vote, several committee members complained that they did not understand the proposal as it was written. Three abstained from the vote.

As the meeting broke up, members could be heard trying to clarify with one another what they had just voted for. The group recast the vote Friday, and elected to align VFC coverage with their recommendation. The combined shot will no longer be covered by the public program.

The committee spent much of its first day debating whether to delay the first dose of the hepatitis B vaccine, a shot typically given at birth, until the child is 1 month old. They will vote on the proposal Friday.

The medical reason for altering the hepatitis B schedule was less clear.

“What is the problem we’re addressing with the hepatitis B discussion? As far as I know, there hasn’t been a spate of adverse outcomes,” said pediatrician Dr. Amy Middleman, one of several people to raise the point during the discussion and public comment period.

Committee member Dr. Robert Malone replied that changing the recommendation for when children should get vaccinated for hepatitis B would improve Americans’ trust in public health messaging.

“A significant population of the United States has significant concerns about vaccine policy and about vaccine mandates, [particularly] the immediate provision of this vaccine at the time of birth,” Malone said. The issue, he said, “is not one of safety, but one of trust.”

Hepatitis B is often asymptomatic, and half of infected people don’t know they have it, according to the CDC. Up to 85% of babies born to infected mothers become infected themselves, and the risk of long-term hazards from the disease is higher the earlier the infection is acquired.

Infants infected with the hepatitis B virus in the first year of life have a 90% chance of developing chronic disease, and 25% of those who do will die from it, according to the the American Academy of Pediatrics.

Since the vaccine was introduced in 1991, infant hepatitis B infections have dropped by 95% in the U.S. Nearly 14,000 children acquired hepatitis B infections from 1990 to 2002, according to the CDC; today, new annual infections in children are close to zero.

This week’s two-day meeting is the second time the committee has met since Kennedy fired all 17 previous ACIP members in June, in what he described as a “clean sweep [that] is necessary to reestablish public confidence in vaccine science.”

The next day, he named seven new members to the committee, and added the last five earlier this week. The new members include doctors with relevant experience in pediatrics, immunology and public health, as well as several people who have been outspoken vaccine skeptics or been criticized for spreading medical misinformation.

They include Pebsworth, whose organization has a long history of sharing inaccurate and misleading information about vaccines, and Malone, a vaccinologist who contributed to early mRNA research but has since made a number of false and discredited assertions about flu and COVID-19 shots.

In some cases, the new ACIP members also lack medical or public health experience of any kind. Retsef Levi, for example, is a professor of operations management at MIT with no biomedical or clinical degree who has nonetheless been an outspoken critic of vaccines.

“Appointing members of anti-vaccine groups to policy-setting committees at the CDC and FDA elevates them from the fringe to the mainstream. They are not just at the table, which would be bad enough; they are in charge,” said Seth Kalichman, a University of Connecticut psychologist who has studied the vaccine information center’s role in spreading vaccine misinformation. “It’s a worst-case scenario.”

Though ACIP holds three public meetings per year, it typically works year-round, said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and a former ACIP member in the early 2000s.

New recommendations to the vaccine schedule are typically written before ACIP meetings in consultation with expert working groups that advise committee members year-round, Offit said. But in August, medical groups including the American Medical Assn., the American Academy of Pediatrics and the Infectious Diseases Society of America were told they were no longer invited to review scientific evidence and advise the committee in advance of the meeting.

That same month, Kennedy fired CDC Director Susan Monarez — who had been appointed to the position by President Trump and confirmed by the Senate. On Wednesday, Monarez told a Senate committee that Kennedy fired her in part because she refused to sign off on changes he planned to make to the vaccine schedule this month without seeing scientific evidence for them.

She did not specify during the hearing what those changes would be.

The ACIP’s recommendations become official only after the CDC director approves them. With Monarez out, that responsibility now goes to Health and Human Services Deputy Secretary Jim O’Neill, who is serving as the CDC’s acting director.

Asked by reporters Wednesday whether the U.S. public should trust any changes the ACIP recommends to the childhood immunization schedule, Sen. Bill Cassidy (R–La.) was blunt: “No.”

Cassidy chairs the Senate committee that oversees the Department of Health and Human Services, and cast the deciding vote for Kennedy’s nomination. Before running for office, Cassidy, a doctor and liver specialist, created a public-private partnership providing no-cost hepatitis B vaccinations for 36,000 Louisiana children.

He cast his vote after Kennedy privately pledged to Cassidy that he would maintain the CDC immunization schedule.

As public trust in the integrity of CDC guidelines wobbles, alternative sources for information have stepped up. Earlier this year, the American Academy of Pediatrics announced that it would publish its own evidence-based vaccination schedule that differs from the CDC’s on flu and COVID shots. And on Wednesday, Gov. Gavin Newsom signed a law giving California the power to establish its own immunization schedule, the same day the state partnered with Oregon and Washington to issue joint recommendations for COVID-19, flu and RSV vaccines.

On Tuesday, an association representing many U.S. health insurers announced that its members would continue to cover all vaccines recommended by the previous ACIP — regardless of what happened at Thursday’s meeting — through the end of 2026.

“While health plans continue to operate in an environment shaped by federal and state laws, as well as program and customer requirements, the evidence-based approach to coverage of immunizations will remain consistent,” America’s Health Insurance Plans said in a statement. The group includes major insurers Aetna, Humana, Kaiser Permanente, Cigna and several Blue Cross and Blue Shield groups. UnitedHealthcare, the nation’s largest insurer, is not a member.

It’s unclear what will be covered after 2026.