The new vaccine board hastily arranged by the health and human services secretary, Robert F. Kennedy, Jr., voted Thursday to stop recommending single-dose flu vaccines containing a mercury-based preservative for both adults and children.

It was the second day of meetings of the Advisory Committee on Immunization Practices, or ACIP, assembled just days before this week’s session. Five members of the seven-member panel voted in favor of recommending single-dose formulations that are free of thimerosal to pregnant women, children 18 years and younger, and adults. 

Only one ACIP board member, the head of the pediatric infectious disease service at Tufts Medical Center, Dr. Cody Meissner, voted “no.”

Another member, a regional director for the National Association of Catholic Nurses, Vicky Pebsworth, abstained.

“The risk from influenza is so much greater than the nonexistent, as far as we know, risk from thimerosal,” Dr. Meissner said. “So I would hate for a person not to receive the influenza vaccine because the only available preparation contains thimerosal. I find that very hard to justify.”

Five other members, including ACIP co-chairs Dr. Martin Kulldorff and Dr. Robert W. Malone, both of whom have been outspoken in their skepticism of COVID-19 vaccine mandates, voted “yes.”

Thimerosal, which is used in flu vaccines to prevent microbial growth, has been in the crosshairs of vaccine critics for years. Mr. Kennedy himself speculated in a 2014 book that it is “immensely toxic to brain tissue” and could cause neurodevelopmental disorders like autism and ADHD, although those claims have been repeatedly debunked. 

On Tuesday, the Centers for Disease Control and Prevention released a new report on thimerosal and neurodevelopmental outcomes, saying it “found no evidence of harm from the use of thimerosal as a vaccine preservative, other than local hypersensitivity reactions.” That report was later removed from the CDC’s website because the “article was not authorized by the Office of the Secretary,” Dr. Malone said during Thursday’s meeting.

Thimerosal has been used in multi-dose vaccines since the 1930s and is different from other toxic kinds of mercury, like the ones found in seafood. The amount of ethylmercury per vaccine dose was considered small and showed little evidence of harm, but the United States started to phase out thimerosal from childhood vaccines in 1999.

Today, thimerosal appears only in the nearly 4 percent of flu doses in the United States that come in multi-dose vials, according to STAT News. During the last fall and winter season, an estimated 96 percent of all flu vaccines administered in the United States were thimerosal-free, according to the report that was removed from the CDC’s website.

However use of the substance was challenged in a controversial report to the panel from Lyn Redwood, the president emerita of anti-vaccine nonprofit Children’s Health Defense. Redwood said she delivered her report, “Thimerosal as a Vaccine Preservative,” in her capacity as a “private citizen.” 

“The evidence for its reproductive toxicity includes severe mental retardation or malformations in human offspring who were poisoned when their mothers were exposed to ethyl mercury or thimerosal when pregnant,” Ms. Redwood read from her presentation. 

Earlier this week, Ms. Redwood’s presentation was abruptly removed from the CDC’s website after it had cited a nonexistent 2008 study to support claims regarding the risk of thimerosal. 

“Removing a known neurotoxin from being injected into our most vulnerable population is a good place to start with making America healthy again,” Ms. Redwood said.

Several studies by groups like the CDC, World Health Organization, and the National Academy of Medicine found no links between autism and vaccines containing thimerosal. 

Dr. Meissner seemed flummoxed by Ms. Redwood’s claims. 

“I’m not quite sure how to respond to this presentation. This is an old issue that has been addressed in the past,” Dr. Meissner said in response. He added that it was important to remember that thimerosal is metabolized into ethyl mercury, which is “excreted much more quickly from the body.”

“It is not associated with the high neurotoxicity that methyl mercury is,” Dr. Meissner said of the neurotoxin commonly found in fish and shellfish.

Dr. Jason Goldman, president of the American College of Physicians, also questioned the panel’s reliance on Ms. Redwood’s presentation.

 “Will there be an actual CDC presentation done by staff scientists, physicians, and those who are subject matter experts with accurate, peer-reviewed scientific data … for the committee to review? Or will we have layperson presentations only?” he asked. 

Ms. Redwood is reportedly expected to start a new position as a special government employee in the CDC’s vaccine safety office sometime soon. 

In a less expected move, the new ACIP  panel voted in favor of recommending an RSV shot for infants. The recommendation provides for a single dose of clesrovimab, a monoclonal antibody, for use in infants whose mothers are not protected against maternal respiratory syncytial virus, or RSV. 

During the first day of ACIP meetings on Wednesday, co-chair Dr. Martin Kulldorff announced the establishment of a new working group that will analyze the current children’s vaccine schedule and look at the “interaction effects” between the various vaccines.

“The number of vaccines that our children and adolescents receive today exceeds what children in most other developed nations receive and what most of us in this room received when we were children,” Dr. Kulldorff said Wednesday. 

Dr. Kulldorff said the group would also look at how two shots, one for hepatitis B and another for the measles, mumps, rubella, and varicella vaccine, or MMRV, are administered to “stay true to evidence-based medicine.”

The American Academy of Pediatrics, or AAP, criticized Thursday’s ACIP meeting, saying in a statement on X that it was “full of the same intentionally misleading fearmongering that causes vaccine hesitancy.”

“This meeting showcased an ACIP that has drifted far from its longstanding focus on science, evidence, and public health. When that focus returns, so will the AAP,” said a member of the AAP Committee on Infectious Diseases, Dr. Adam Ratner, in a statement posted on X. 

Mr. Kennedy defended his wholesale firing of all 17 members of the previous ACIP board, saying they were “plagued with conflicts of interest.” 

At one point during Thursday’s meeting, Ms. Pebsworth asked to read a statement to disclose that she owned shares in a healthcare sector fund, which “includes holdings relevant to the ACIP, including vaccine manufacturers.”

“I don’t know whether I have a conflict or not,” Ms. Pebsworth said.

In a statement, the ACIP board defended its decisions, saying its votes are “recommendations.”

“We came to this meeting with no pre-determined ideas and will make judgments as if we are treating our own families. Unbiased scientific thinking is fundamental to the committee’s charge,” the statement read.