The Controversy Over Thimerosal in Vaccines: A Historical Overview

A century ago, bacterial contamination was a major safety risk in vaccines. In 1916, four children in South Carolina died from a typhoid vaccine tainted with Staphylococcus aureus. Similarly, in 1928, 12 children in Queensland, Australia, lost their lives due to contaminated diphtheria immunizations.

By the 1930s, vaccine manufacturers introduced thimerosal, a mercury-based preservative, to prevent microbial growth. For decades, it was considered safe, with only minor side effects like localized reactions at the injection site, according to the FDA.

However, concerns arose in 1999 when U.S. health officials recommended removing thimerosal from vaccines. Although no direct harm had been proven, its mercury content raised theoretical concerns about potential neurotoxicity in children. Since then, thimerosal has been phased out of most vaccines, except for certain multi-dose flu shots. Subsequent research has found no link between thimerosal and neurodevelopmental disorders, including autism—yet autism rates have continued to climb.

Given this settled science, public health experts were surprised when thimerosal appeared on the agenda for this week’s meeting of the CDC’s vaccine advisory committee.

A Shake-Up in the Advisory Panel
The meeting marks the first since Robert F. Kennedy Jr., a prominent vaccine skeptic, dismissed all 17 previous committee members, alleging conflicts of interest. He replaced them with eight new advisers, some of whose qualifications have been questioned. Just before the meeting, one member withdrew during a financial review, leaving the panel with seven members.

On Thursday, the committee is set to discuss thimerosal in vaccines, with a focus on flu shots, followed by a possible vote on recommendations. If approved, these could influence CDC policy, pending approval from agency leadership or, in the absence of a confirmed director, from Kennedy himself.

Why Thimerosal Is Back in the Spotlight
Vaccine experts see the sudden focus on thimerosal as alarming, given the overwhelming scientific consensus on its safety. They worry that Kennedy—who once led the anti-vaccine group Children’s Health Defense—is reigniting unfounded fears. Adding to concerns, Lyn Redwood, another former leader of the same organization, is scheduled to present on the topic.

This renewed debate raises questions about why a long-resolved issue is being revisited—and what it means for public trust in vaccines.