Why Is Thimerosal Back On The Cdc's Agenda When It's Barely In Flu Shots Anymore?

An influential panel of CDC advisors is slated to vote on a little-known preservative called thimerosal -- a move that leaves many vaccine scientists scratching their heads.

Thimerosal is an ingredient that helps keep vaccines pathogen free. Health and Human Services Secretary Robert F. Kennedy Jr., a noted vaccine skeptic, has falsely stated the ingredient is a contributing factor in causing autism. In recent decades, the preservative largely has been phased out of vaccines, even as autism rates continue to climb.

On Wednesday, a panel of eight CDC advisors hand-picked by Kennedy is slated to vote on whether the ingredient should be banned completely. Lyn Redwood, a nurse practitioner who is the former president of Children's Health Defense, an anti-vaccine group founded by Kennedy, is slated to present before a vote on the use of thimerosal. As the agenda item recently added it's not clear what the discussion and vote will include.

Vaccine specialists interviewed by ABC News said the vote on thimerosal was surprising given most flu shots no longer contain it, and they worried the vote could reinforce the false idea that thimerosal is not safe.

The CDC's Advisory Committee on Immunization Practices (ACIP) is the federal body that advises the CDC on vaccine use in the United States. The committee's decisions shape public health policy and influence vaccine recommendations nationwide.

With no confirmed CDC director in place, Kennedy Jr. has taken a leading role by overseeing the agency’s direction, appointing members to the vaccine advisory panel and chairing its public meetings. The White House has named a CDC director nominee, Susan Monarez, who has been serving as the acting director since January, but the confirmation process won’t be complete in time for this week’s advisory panel meeting.

Thimerosal is a preservative introduced in the 1930s that contains ethylmercury, a compound chemically related to mercury but processed differently by the body. Nevertheless, vaccine skeptics have falsely suggested it could be contributing to autism.

It helps prevent bacterial and fungal growth in multidose vials that are punctured repeatedly to vaccinate multiple people.

According to the CDC's most updated information, about 94% of flu vaccines distributed in the U.S. this season are thimerosal free or completely preservative free.

The topic, added late to the ACIP agenda and without much detail, has baffled some vaccine experts.

"It just isn't making sense to me," said Dr. Jason Goldman, president of the American College of Physicians and a liaison member to ACIP. "It's just a mystery to me why they're even having this on the agenda."

A spokesperson for the Department of Health and Human Services did not immediately respond to ABC News' request for comment.

Goldman, who is invited to the meeting, said he thought thimerosal's limited use makes the renewed focus hard to justify.

"It's only a small percentage of the vaccines that still have a preservative," he said, adding that he personally does not use multi-dose vials because he doesn't see much difference between multi-dose and single-dose shots.

And thimerosal is not just rare in U.S. flu vaccines—it's also broadly considered safe, Goldman noted.

"Autism is not caused by vaccines," Goldman said. "We have proven that time and time again."

A nurse prepares to administer a flu shot during a 9th Ward COVID-19 and flu vaccination clinic at the Pullman Community Center in Chicago on Oct. 10, 2024.

Tess Crowley/Chicago Tribune/Tribune News Service via Getty Images

Dr. Mark Abdelmalek, an ABC News medical contributor who has written extensively about vaccines and hosts a podcast that often focuses on vaccine-policy related issues, agreed.

"Thimerosal has been proven safe and there is no evidence to suggest thimerosal causes autism," he said.

However, some groups continue to claim that thimerosal causes autism.

Even though so few flu vaccines still contain the preservative, Abdelmalek cautioned that taking it out of the remaining supply could compromise safety and access for more remote, far-away clinics that rely on multi-dose vials.

"If you removed thimerosal from a multi-dose vial, without replacing it with another preservative, it could actually make the vaccines less safe," he said.

Multi-dose vials are still used in global vaccination programs and in lower-resource U.S. clinics, where they use less storage space, and allow for faster vaccination, Abdelmalek explained.

"Some form of preservative is necessary for multi-dose vaccine vials," he said. "The idea of eliminating all preservatives from vaccines simply isn't realistic or safe for that matter."

Goldman questioned the motives behind reintroducing the topic at an official government meeting. An ACIP vote to remove thimerosal from flu vaccines could signal policy change, but it won't alter the vaccine options for most Americans, he added.

"I don't see how it's that substantive of an impact if it gets rid of multi-dose and just keeps the single-dose pre-filled syringe," he said. "It seems like delay, delay, delay.… Maybe to further sow doubt about vaccines in general."