4 things to know about the vaccine ingredient thimerosal

Though astir flu vaccines don't see nan preservative thimerosal, advisers to nan CDC person recommended against utilizing it. Marie D. De Jesus/Houston Chronicle/Hearst Newspapers/Getty Images hide caption

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Marie D. De Jesus/Houston Chronicle/Hearst Newspapers/Getty Images

The national authorities could soon urge that group only person flu shots made without an constituent called thimerosal.

The preservative has been absent from nan mostly of flu vaccines for astir 2 decades, but was connected nan schedule of a committee that advises nan Centers for Disease Control and Prevention connected vaccine policy.

The June gathering was nan first since earlier successful nan month, erstwhile Health Secretary Robert F. Kennedy Jr. sacked each 17 erstwhile members of nan Advisory Committee connected Immunization Practices, aliases ACIP, and installed his ain slate of seven.

Thimerosal has been a target of groups that mobility vaccine safety. That's contempt "a agelong grounds of safe and effective use" arsenic a vaccine preservative, according to nan Food and Drug Administration.

In 3 abstracted votes, ACIP recommended that children, pregnant women and each adults person single-dose flu immunizations pinch vaccines that don't incorporate thimerosal. The adjacent measurement would beryllium a determination by Kennedy aliases nan CDC connected nan recommendations.

"It's benignant of for illustration they turned this gathering into solving a problem that doesn't really beryllium anymore," says Dr. Jesse Goodman, a erstwhile main intelligence astatine FDA who is now astatine Georgetown University.

The lone committee personnel to ballot against nan recommendations was Dr. Cody Meissner, a professor of pediatrics astatine Dartmouth College.

"Of each nan issues that ACIP needs to attraction on, this is not a large issue," he said. "The consequence from influenza is truthful overmuch greater than nan non-existent, arsenic acold arsenic we know, consequence from thimerosal."

He added: "There is nary technological grounds that thimerosal has caused a problem."

But what is thimerosal and what would hap if it is efficaciously banned from flu shots? Here are 4 things to know.

1. It's a preservative that contains mercury

Thimerosal is simply a chemic compound that's astir 50% mercury by weight, according to nan FDA, and has been utilized successful trace amounts arsenic a preservative successful vaccines and medicines since nan 1930s.

The compound is utilized successful vials containing aggregate doses of nan flu vaccine. Its occupation is to support nan vial from getting contaminated betwixt patients and perchance getting personification sick, says Dr. Michelle Fiscus, main objective serviceman of nan Association of Immunization Managers.

"If you're perpetually going successful and retired of nan vial to tie up a caller dose, you tally nan consequence of getting germs aliases fungus into that vial," says Fiscus. "And we don't want those vaccines to beryllium contaminated."

She says though astir flu vaccines travel successful single-dose prefilled syringes today, nan multi-use vials are cheaper and return up little abstraction successful refrigerators. So for immoderate wellness centers, they're much practical.

"When we're coming into flu season, it's not uncommon to get each of your flu vaccine upfront astatine nan opening of nan season. And sometimes retention successful nan refrigerator tin beryllium an issue," Fiscus says.

Still, they're not common. According to nan CDC, 94% of flu shots successful nan 2024-25 play were thimerosal-free aliases thimerosal-reduced.

2. Thimerosal has been utilized infrequently since 2001

In 1997, nan FDA Modernization Act required nan authorities to measure mercury successful puerility vaccines.

But nan dense metallic comes successful different forms. Methylmercury is nan compound recovered successful seafood that doesn't break down easy and tin beryllium toxic. Ethylmercury, nan shape successful thimerosal, has a different consequence profile.

"It didn't separate betwixt that benignant of mercury and ethylmercury, which is simply a shape of mercury that's very quickly managed by nan assemblage and eliminated done nan kidneys," says Fiscus. "And ethylmercury is wherever thimerosal is derived."

Thimerosal was removed from astir puerility vaccines successful 2001, moreover though location was nary grounds showing it caused harm to vaccine recipients, Fiscus says.

3. It has a discredited nexus to autism 

Concerns that vaccines origin autism originated pinch a now-retracted 1998 study by Dr. Andrew Wakefield, which focused connected nan MMR vaccine against measles, mumps and rubella. Wakefield mislaid his aesculapian licence and The BMJ, a salient aesculapian journal, called it an "elaborate fraud" successful a 2011 editorial.

Despite that, speculation astir a nexus betwixt vaccines and autism — erstwhile championed by Kennedy himself — continued and expanded to see thimerosal, moreover though a nexus to autism has been many times disproven.  

A 2004 report published by nan National Academies of Sciences Engineering and Medicine reiterated that thimerosal wasn't associated pinch autism. Subsequent studies successful 2006, 2007, 2009 and 2010 showed thimerosal was not associated pinch autism aliases neurophysiological problems, according to nan CDC.

What's more, Georgetown's Goodman points retired that contempt thimerosal's removal from astir vaccines, autism rates person continued to emergence complete nan past 20 years, "which surely would not beryllium accordant pinch having thing to do pinch puerility vaccines."

4. Most group won't announcement if thimerosal is nary longer recommended successful shots

Since astir flu shots are administered pinch single-dose syringes, astir group astir apt won't announcement this autumn and wintertime if nan CDC decides to efficaciously prohibition thimerosal, Fiscus says.

Still, a thimerosal prohibition could coming immoderate challenges because wellness centers statesman ordering flu vaccines successful February for nan adjacent flu season. It's imaginable manufacturers will person to switch them out.

Sanofi, which makes multidose vials of a vaccine called Fluzone arsenic good arsenic single-dose syringes, says only a "very mini number" of its doses incorporate thimerosal. "We admit nan proposal of nan caller ACIP. We now await nan determination by nan CDC connected nan way forward. We will person capable proviso of Sanofi flu vaccine to support customer penchant for this season."

Seqirus, nan different institution that makes multidose vials of flu vaccine containing thimerosal, said successful a connection to NPR that they "represent a very mini proportion" of its full vaccine supply. "We are committed to supporting our customers successful afloat transitioning to single-dose syringes and do not expect immoderate impacts to our proviso aliases shipment timing this season."

The committee's process could beryllium a motion of things to come

Fiscus, from nan Association of Immunization Managers, says nan committee's determination to only urge single-dose flu shots without thimerosal shows that it is consenting to make a determination without pursuing protocol and considering nan technological evidence.

"Is this now going to beryllium nan standard?" she says. "That's very concerning if that's wherever this is heading."

Goodman besides worries that these moves will yet undermine assurance successful vaccines successful nan United States and abroad.

"One of my existent concerns is that though nan flu vaccines aren't perfect, they prevention lives. When they're given, they're still underutilized," he says. "If there's much trouble successful accessing them aliases affording them, astatine nan extremity of nan day, group would get hurt."