The panel, known as the Advisory Committee on Immunization Practices, is also considering a change to hepatitis B shots, which are now given to infants on the day they are born. On Friday, it's expected to decide whether to recommend some parents wait one month for their infants to receive those shots.

Also on Friday, the committee is expected to consider changes to recommendations for the COVID-19 vaccine, after Kennedy had already moved to limit access to those shots.

The committee advises the Centers for Disease Control and Prevention on changes to the federal government's standing guidance on vaccines. The agency almost always accepts those recommendations, which are widely heeded by doctors and guide vaccination programs.

But the annual review of vaccines has become politically supercharged under Kennedy, a longtime skeptic who has attacked many vaccines and earlier this year fired all 17 members of the advisory panel, replacing them with a group that includes several antivaccine voices.

Kennedy's actions prompted Massachusetts among other blue states to break with federal policy. The Healey administration has declared that insurers and pharmacies must follow vaccine policy as laid out by the Massachusetts Department of Public Health; for example, the state adopted a more expansive view on COVID shots that largely mirrors the federal stance before Kennedy took office. So the CDC advisory committee's recommendations will not affect when Massachusetts residents can get those vaccines.

In an interview Thursday, Massachusetts public health commissioner Dr. Robbie Goldstein noted that even though vaccines for measles, mumps, rubella, and chickenpox would still be available regardless of the committee's decisions, there could still be value in having the combined dose available for younger children.

He would consider how removing access to the combined shot could affect access in Massachusetts and the cost to families.

"It's an area where I want to make sure we put it through a formal process," he said. "I want to make sure we are reviewing all the data."

But public health experts worry the changes at the federal level will raise unwarranted concerns among parents. Kennedy, a leading antivaccine activist before becoming the nation's top health official, has made or proposed numerous changes to the nation's vaccine system.

Discussions on the MMRV vaccine focused largely on rare instances of feverish seizures associated with the first dose that is currently given to kids between ages 1 and 2.

Committee member Dr. Cody Meissner said such seizures may be "a very frightening experience" for families, but medical experts agree they're not linked to brain function or school problems.

The panel last dealt with the issue in 2009, when it said that either the combination shot or separate MMR and varicella shots were acceptable for the first dose, but that separate doses were generally preferred. Today, 85 percent of children receive separate doses for the first round, according to information presented at the meeting.

Some doctors and public health experts say they are not aware of any new safety data that would explain revisiting those vaccination recommendations. In fact, many of the studies discussed Thursday were more than a decade old.

Dr. Richard Haupt, a vice president at Merck, which makes the MMRV vaccine ProQuad, said it's been evaluated through clinical trials and post-approval studies, and the slight increase in feverish seizures after the first dose led to current CDC recommendations. Combination vaccines improve completion and on-time vaccination at a time when the nation is seeing a troubling decline in vaccination coverage, he said.

"Considering these trends, any policy decision that compromises the clarity or consistency of vaccination guidance . . . has the potential to further diminish public confidence," he told the committee.

Meanwhile, the potential change to the hepatitis B vaccine is troubling to Dr. Charles Hannum, a general pediatrician at Tufts Medical Center. He said giving the shot immediately after birth is critically important since infants are at high risk of infection through exposure to an infected mother's blood during labor.

"Changing the schedule may make this harder for patients to receive the hepatitis B vaccine in the hospital, when it would be the best time to give the first dose and prevent perinatal infection," Hannum said. Waiting even a month, as the panel is considering, would increase the risk of a child developing an infection that can lead to liver disease, liver cancer, cirrhosis, and liver failure, he said. About 90 percent of infants infected in their first year will develop chronic hepatitis B.

"The risk of morbidity is exceptionally high for infants," said Hannum.

Vaccines have been dramatically effective at reducing the instance of hepatitis B in the United States, with cases dropping 99 percent from 1990 to 2019, according to the National Institutes of Health.

In adults, the virus is spread through sex or through sharing needles during injection-drug use. But it can also be passed to a baby from an infected mother. The virus can also live on surfaces for more then seven days at room temperature, and unvaccinated children living with anyone with a chronic infection is at risk of infection.

During Thursday's discussion, some committee members questioned whether babies born to moms who test positive for hepatitis B are the only ones who truly need a vaccine in the first day of life.

By giving virtually all babies the hepatitis B right after birth, "Are we asking our babies to solve an adult problem?" asked committee member Dr. Evelyn Griffin.

But Meissner expressed bewilderment at some of this discussion.

"This is an absolutely safe vaccine," he said. "I'm not sure what we're gaining by avoiding that first dose within 12 to 24 hours after birth."

He later added: "We will be creating new doubts in the mind of the public that are not justified."

In his opening remarks, the new chairman of the advisory committee, Martin Kulldorff, defended the group against critics who say it leans toward anti-vaccine views.

"The members of this ACIP Committee are committed to reassuring the public and restoring public confidence by removing unnecessary risks and harms whenever possible. That is a pro-vaccine agenda," Kulldorff said.

He later added: "We welcome scientific critique of any of our votes, as there are gray areas due to incomplete scientific knowledge."

Kennedy's advisory panel recommends new restrictions on MMRV vaccines - The Boston Globe