Context:

The Trump administration's vaccine advisers have revisited the debate over the use of thimerosal, a preservative in some flu vaccines, despite studies showing no health risks associated with it. Thimerosal, containing ethylmercury, has been used since the 1930s to prevent bacterial contamination in multi-dose vaccine vials, but it was mostly phased out from children's vaccines by 2001. The ongoing concern, largely fueled by antivaccine activists like Robert F. Kennedy Jr., stems from unproven claims linking thimerosal to autism, even though autism rates continued to rise after its removal. Most flu vaccines in the U.S. are now thimerosal-free, with 96% of those administered last year lacking the preservative. CDC analyses continue to show no connection between thimerosal and autism or other neurodevelopmental disorders, yet the debate persists over its presence in the remaining flu shots for adults and children.

Dive Deeper:

• Thimerosal is a preservative that has been used in vaccines since the 1930s to prevent bacterial contamination, particularly in multi-dose vials. It contains ethylmercury, which is different from the more toxic methylmercury found in seafood, and is efficiently excreted by the body.

• Concerns about thimerosal arose in the late 1990s due to its mercury content, leading to a phase-out of the preservative in U.S. childhood vaccines by 2001. Despite this, scientific studies have consistently shown no evidence of harm from thimerosal, and it was never used in vaccines for diseases like chickenpox, polio, or measles.

• The debate over thimerosal re-emerged with the Trump administration's vaccine advisers considering its complete removal from the few remaining flu vaccines that still contain it. This discussion is fueled by long-standing but unfounded claims linking thimerosal to autism, promoted by figures like Robert F. Kennedy Jr.

• Currently, the vast majority of flu vaccines in the U.S., including 96% of those administered last fall and winter, do not contain thimerosal, especially those used in federally funded programs. Most children receive their flu vaccinations from single-dose shots that are thimerosal-free.

• A recent CDC analysis prepared for a vaccine advisers' meeting reconfirmed that there is no link between thimerosal and autism or other neurodevelopmental disorders. Despite this, some continue to argue for the removal of thimerosal in the remaining vaccines, citing potential risks to specific groups such as pregnant women.