The Department of Health and Human Services (HHS) has adopted a recommendation to remove thimerosal, a mercury-based preservative, from all influenza vaccines in the United States.

A 2025 advisory panel for the Centers for Disease Control and Prevention (CDC) made the recommendation, even though there’s no clear evidence of harm and most flu shots in the U.S. are free of thimerosal.

What Is Thimerosal and Why Is It Used in Vaccines?

Thimerosal is a mercury-containing preservative used to prevent bacterial growth in multi-dose vials of vaccines and medicines.

Decades ago, some reports raised concerns about thimerosal’s potential link to developmental and cognitive issues in children, including autism.

No actual studies confirm the link, but thimerosal was removed from most vaccines in 1999 “out of an abundance of caution,” said Robert Hopkins Jr, MD, medical director of the National Foundation for Infectious Diseases.

Nearly all vaccines are available in thimerosal-free versions. About 94% of the U.S. flu vaccine supply does not contain thimerosal; those that do are typically found in multi-dose vials used in some rural clinics.

“The data has consistently shown that thimerosal is not harmful in the tiny amounts previously used, and today, most patients can easily avoid vaccines that contain it,” Hopkins said.

Is Thimerosal Safe?

There are two types of mercury. Methylmercury, which is found in some fish, can be harmful to humans in high doses. Ethylmercury—the type used in thimerosal—is cleared from the body more quickly, making it less likely to cause any harm.

The CDC states that thimerosal has a strong safety record in medical products, and multiple studies have found no evidence of harm from the low doses used in vaccines.

“This fact was definitively determined years ago, and the FDA, the CDC, and many academic institutions have excellent reviews of the safety of thimerosal-containing vaccines in children. Raising this topic now has only one purpose—to sow distrust in vaccines in general and in the process to determine vaccine safety,” James Campbell, MD, MS, FAAP, a member of the American Academy of Pediatrics Committee on Infectious Diseases, said in a statement.

Hopkins said chemical ingredients in vaccines have long been a focus of anti-vaccine messaging. He often tells concerned patients that while many chemicals are potential toxins, what matters is the amount. “We all know we need iron, but if we get too much, it’s damaging,” he added.

Only one new member of the Advisory Committee on Immunization Practices (ACIP) voted against the recent recommendation on thimerosal.

H. Cody Meissner, MD, a professor of pediatrics at the Dartmouth Geisel School of Medicine, said during the panel, “The ACIP makes recommendations based on scientific evidence as much as possible… and there is no scientific evidence that thimerosal has caused a problem.”

Related Stories

Do You Really Have to Throw Out That Expired Food?

Who Should Get the RSV Vaccine?

Thimerosal Controversy Sparks Concern Over Vaccine Access and Trust

Thimerosal was not on the original agenda of the ACIP meeting.

However, the topic was added after HHS Secretary Robert F. Kennedy Jr. removed all 17 existing ACIP members—who are usually vetted by the CDC—and replaced them with his own appointees. According to news reports, several new members have publicly expressed skepticism about vaccines.

Infectious disease experts are concerned that the ACIP vote could fuel vaccine hesitancy and limit flu shot access, particularly in underserved areas.

Angela Branche, MD, an infectious disease specialist at the University of Rochester, said at a briefing by the Infectious Diseases Society of America that many rural clinics lack space to store single-dose vials.

The change would carry both “a monetary cost for these clinics and a cost in who gets vaccinated,” Branche said.