Lyn Redwood, a nurse practitioner who once ran the anti-vaccine group that Robert F. Kennedy Jr. founded, attends a meeting of the Advisory Committee on Immunization Practices at the Centers for Disease Control and Prevention in Atlanta on Thursday, June 26, 2025. (AP Photo/Shelby Lum)
Lyn Redwood, a nurse practitioner who once ran the anti-vaccine group that Robert F. Kennedy Jr. founded, attends a meeting of the Advisory Committee on Immunization Practices at the Centers for Disease Control and Prevention in Atlanta on Thursday, June 26, 2025. (AP Photo/Shelby Lum)
FILE - A nurse prepares a flu shot from a vaccine vial at the Salvation Army in Atlanta, Feb. 7, 2018. (AP Photo/David Goldman, File)
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Lyn Redwood, a nurse practitioner who once ran the anti-vaccine group that Robert F. Kennedy Jr. founded, attends a meeting of the Advisory Committee on Immunization Practices at the Centers for Disease Control and Prevention in Atlanta on Thursday, June 26, 2025. (AP Photo/Shelby Lum)
Lyn Redwood, a nurse practitioner who once ran the anti-vaccine group that Robert F. Kennedy Jr. founded, attends a meeting of the Advisory Committee on Immunization Practices at the Centers for Disease Control and Prevention in Atlanta on Thursday, June 26, 2025. (AP Photo/Shelby Lum)
The Trump administration’s vaccine advisers took up an old flu-shot debate: whether it’s time to wipe out the last small fraction of those vaccines that contain a controversial preservative called thimerosal.
It’s a question seemingly laid to rest years ago, as studies showed no evidence that the preservative causes any health problems. Yet that vaccine panel recommended Thursday that people should receive only thimerosal-free flu vaccinations this fall — something already easy to do since government data shows only a small share of last year's vaccine contained the preservative.
“This is really a nonissue,” said Dr. Sean O’Leary of the American Academy of Pediatrics, who worries the scrutiny “is sowing distrust” of safe vaccinations.
Here are some things to know about the substance.
What is thimerosal?
Thimerosal is a preservative used in certain vaccines since the 1930s, as well as in some other medical products.
It was mostly used in multi-dose vials of vaccine, to prevent bacterial contamination as the vessel was repeatedly punctured to withdraw a dose.
Why is thimerosal controversial?
Questions about thimerosal were raised in the late 1990s because it contains a form of mercury.
It’s not the same as the toxic type found in some seafood, called methylmercury. Instead, it’s a different type called ethylmercury that the body can excrete, O’Leary explained.
The amount of ethylmercury per vaccine dose was small and studies found no evidence of harm. Nor was it used in all vaccines. For example, vaccines for chickenpox, polio and measles, mumps and rubella never contained it, according to the Centers for Disease Control and Prevention.
But as a precaution, the U.S. phased the preservative out of childhood vaccines. Since 2001, all vaccines routinely recommended for children age 6 and younger in the U.S. come in formulas that don’t contain thimerosal.
The exception is a small subset of flu shot formulas in multi-dose vials that can be used for adults or kids. The vast majority of children, however, get their flu vaccination from a single-dose shot, O’Leary said.
According to the CDC, 96% of all flu vaccines in the U.S. administered last fall and winter — and an even higher share of those used in federal programs like Vaccines for Children — were thimerosal-free.
Why is thimerosal being debated again?
U.S. Health Secretary Robert F. Kennedy Jr. has long contended there was a tie between thimerosal and autism.
On Thursday, his newly appointed vaccine advisers heard a presentation from Lyn Redwood. She is past president of the anti-vaccine group he led for years and was just hired as an “expert” by his agency.
She told the group that thimerosal should be removed from remaining flu shots. Among her claims were that they could be a risk to pregnant women.
A prepared for the meeting — but not allowed to be publicly presented — concluded there was no link between the preservative and autism or other neurodevelopmental disorders. Some experts also note that autism rates rose after thimerosal was removed from young children's vaccines in the U.S.
The preservative is still used in vaccines in other countries.
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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.
