Vaccine advisers recently appointed by Health Secretary Robert F. Kennedy Jr. voted on Thursday to stop recommending flu vaccines containing thimerosal, a mercury-based preservative used to prevent bacterial contamination, to children and pregnant women.
Dozens of studies have shown thimerosal to be harmless, and it has not been a component of most childhood shots since 2001. Yet Mr. Kennedy and other critics have long insisted that the preservative might be linked to rising rates of autism.
“The risk from influenza is so greater than the nonexistent risk as far as we know from thimerosal,” said the lone dissenter, Dr. Cody Meissner, a pediatrician at Tufts Children’s Hospital and widely considered to be the most qualified member of the new committee.
“I find it very hard to justify” the panel’s decision, he added.
In a separate vote, the new advisers recommended seasonal flu vaccines to all Americans 6 months and older.
On the second day of their meeting, the advisers seemed to be warming to their roles as disrupters of the decades-old processes that have guided vaccines to Americans.
In addition to certain flu vaccines, some panelists questioned the safety of other products already approved by the Food and Drug Administration and thoroughly vetted by independent experts.
Others seemed unaware of even basic information about the Vaccines for Children program, which provides immunizations for free to roughly half of all American children.
Insurance companies and government programs like Medicaid and the V.F.C. are required to cover vaccines that are recommended by the panel, called the Advisory Committee on Immunization Practices.
“It’s striking how little the voting members seem to know about the diseases and vaccines that they are discussing,” said Dr. Adam Ratner, a pediatric infectious diseases physician and expert on vaccine policy, as he followed the deliberations.
During the deliberations on a vaccine for respiratory syncytial virus, one of the panelists, Dr. Robert Malone, said that there had been “very, very active discussion and consideration within the committee, just for the record, concerning this product. It has been very actively debated internally.”
Outside experts interpreted that to mean that panelists had been privately discussing the topic among themselves, something that they are not legally allowed to do.
“In other contexts, off-record conversations have been grounds for overturning a decision,” said Dorit Reiss, an expert on vaccine policy and law at the University of California College of the Law, San Francisco.
“Though it reflects lack of understanding of their limits, it’s unlikely to lead to overturning,” she added.
A spokesman from the Department of Health and Human Services said that Dr. Malone was referring to a one-on-one conversation between himself and another panelist, which was “permissible.”
Presentations at the meetings are generally made by C.D.C. staff. But among the speakers on Thursday was Lyn Redwood, a former leader of Children’s Health Defense, the anti-vaccine group founded by Mr. Kennedy.
Ms. Redwood, who has been hired as a special employee at the Department of Health and Human Services, alleged that thimerosal was toxic and dangerous to children.
A scientific review of evidence showing the safety of thimerosal, posted by C.D.C. scientists on Tuesday, was taken down shortly thereafter.
“Giving someone like Lyn Redwood a voice at the A.C.I.P. meeting to spread misinformation about a long-settled matter of vaccine safety is yet another troubling way in which R.F.K. Jr. is inappropriately interfering with the C.D.C. vaccine policy process,” said Dr. Fiona Havers, who resigned last week from her position as a senior C.D.C. adviser on vaccine policy.
On Wednesday, The Times reported that a slide in Ms. Redwood’s presentation incorrectly said that flu vaccine contains 50 micrograms of thimerosal, double the actual amount in the shot.
That slide was removed from the version Ms. Redwood presented on Thursday, but the inaccurate information was repeated in a different slide.
Her presentation had also initially included a slide with references to studies that did not exist; that slide was also removed.
During the 2024-25 respiratory virus season, only 3 percent of children and 2 percent of older adults received flu vaccines containing thimerosal, according to an analysis of electronic health records by Truveta.
Dr. Meissner sharply objected to Ms. Redwood’s comments.
“I’m not quire sure how to respond to this presentation,” he said. “This is an old issue that has been addressed in the past.”
“There is no scientific evidence that thimerosal has caused a problem,” he added.
Liaisons to the A.C.I.P. from several organizations, including the American College of Obstetricians and Gynecologists, also challenged the idea that thimerosal is toxic. The panelists are expected to vote later on Thursday on whether to stop offering flu vaccines containing thimerosal to children and pregnant women.
In another unusual move, Martin Kulldorff, the panel’s chair, proposed that the members vote to stop allowing use of a vaccine for measles, mumps, rubella and chickenpox among children under 4. It has been available since 2005.
This combination vaccine slightly increases the risk of febrile seizures, which can occur with any childhood illness but do not cause lasting harm. The C.D.C. already recommends administering the M.M.R. shot and the chickenpox shot separately in the first dose. The panelists are expected to vote on the vaccine’s use at a future meeting.
The committee’s very first vote of the day suggested that vaccines and other drugs are likely to receive skeptical reviews from the reconstituted A.C.I.P.
Clesrovimab is a new monoclonal antibody approved by the F.D.A. earlier this month to protect infants from R.S.V. infection. The committee’s vote on recommending it was expected to be a straightforward, as clesrovimab is not technically a vaccine.
But a new panel member, Retsef Levi, a professor of operations science at the Massachusetts Institute of Technology, said that he would like to discuss some data he “had found” from clinical trials that raised concerns about deaths in children receiving the product.
Experts from the C.D.C. and the F.D.A., and the vaccine manufacturer Merck, all said the data had been reviewed very carefully before the antibody’s approval and did not reveal any association between the deaths and the product.
Experts watching the proceedings were incensed. “These trials have been discussed at length at prior meetings, and the questions were answered to the satisfaction of all of the members,” said Dr. Sean O’Leary, the chair of the infectious disease committee at the American Academy of Pediatrics, and a member of the R.S.V. work group convened by the C.D.C.
The panelist Dr. Meissner noted that a work group of 60 experts, including himself, had reviewed the safety data “very, very carefully.” He defended both the monoclonal antibody and an R.S.V. vaccine for pregnant women as valuable new tools. R.S.V. is the leading cause of hospitalizations in infants.
“We have come so far in the last few years in making these products available, and really it’s a result of the extraordinary investigations,” Dr. Meissner said.
Safety, he added, “is simply not an issue here.”
Another monoclonal antibody, nirsevimab, sharply reduced R.S.V. hospitalizations among infants during the 2024-25 season, and it so far does not appear to pose safety concerns apart from some cases of hives, according to data presented at the meeting on Wednesday.
Dr. Levi and Vicky Pebsworth, a nurse affiliated with an anti-vaccine group, voted against recommending clesrovimab; the other five members voted yes. After some confusion, the panel also voted to include the antibody in the children’s vaccine program.
Some panelists also raised concerns about a flu vaccine that would be offered to everyone ages 9 and older. As of June 14, flu was associated with 250 deaths in children in the United States, according to data presented on Thursday.
Dr. Malone called that a “modest number,” but some experts noted that the number of deaths were the highest in a nonpandemic year in a long time, and almost certainly an undercount.
Apoorva Mandavilli reports on science and global health for The Times, with a focus on infectious diseases and pandemics and the public health agencies that try to manage them.
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