The Trump administration’s vaccine advisers are bringing 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.

“This is really a nonissue,” said Dr. Sean O’Leary of the American Academy of Pediatrics.

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 could be used for adults or kids. The vast majority of children, however, get their flu vaccination from a single-dose shot instead, 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 federally funded programs — were thimerosal-free.

Why is thimerosal being debated again?

U.S. Health Secretary Robert F. Kennedy Jr. is a longtime antivaccine activist, and he has long contended there was a tie between thimerosal and autism.

On Thursday, his newly appointed vaccine advisers were set to hear a presentation from Lyn Redwood, a nurse practitioner who once ran the antivaccine group that Kennedy founded.

She will suggest that the remaining thimerosal-containing flu shots should be removed, according to meeting materials posted earlier this week. Among the claims were that they could be a risk to pregnant women.

A new CDC staff analysis prepared for the meeting again showed no link between the preservative and autism or any other neurodevelopmental disorders.

Some experts note that autism rates rose after thimerosal was removed from young children’s vaccines in the U.S.

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