The federal government is poised to recommend that individuals exclusively receive flu shots free from an ingredient known as thimerosal. This preservative has been largely absent from the majority of flu vaccines for nearly two decades. However, it was a topic of discussion during a recent meeting of a committee that advises the Centers for Disease Control and Prevention (CDC) on vaccine policies. This meeting was notably the first since Health Secretary Robert F. Kennedy Jr. dismissed all previous members of the Advisory Committee on Immunization Practices (ACIP) and appointed a new group of seven members.
Thimerosal has been a focal point for groups questioning vaccine safety, despite its long history of safe and effective use as a preservative, as acknowledged by the Food and Drug Administration (FDA). During the recent ACIP meeting, members voted to recommend that children, pregnant women, and all adults receive single-dose flu vaccinations that do not contain thimerosal. The next crucial step involves a decision by Secretary Kennedy or the CDC regarding these recommendations.
Dr. Jesse Goodman, a former chief scientist at the FDA and current expert at Georgetown University, expressed concerns about the committee's focus on thimerosal. "It's as if they turned this meeting into solving a problem that doesn't really exist anymore," he remarked. The only committee member to oppose the recommendations was Dr. Cody Meissner, a professor of pediatrics at Dartmouth College, who stated that the risk posed by influenza significantly outweighs any perceived risk from thimerosal.
Thimerosal is a chemical compound composed of approximately 50% mercury by weight, according to the FDA. Used since the 1930s, it serves as a preservative in vaccines and medicines. The compound is particularly useful in vials containing multiple doses of the flu vaccine. Dr. Michelle Fiscus, chief clinical officer of the Association of Immunization Managers, explained that thimerosal helps prevent contamination. "If you're frequently accessing the vial to draw a new dose, you risk introducing bacteria or fungi, which could jeopardize patient safety," she stated.
While most flu vaccines are now available in single-dose prefilled syringes, some health centers still prefer multi-use vials due to their cost-effectiveness and space-saving benefits. "At the beginning of flu season, it's common for health centers to order all their vaccines at once, which can lead to storage challenges," Fiscus added. Currently, the CDC reports that around 94% of flu shots in the 2024-25 season are either thimerosal-free or contain reduced levels of thimerosal.
Thimerosal's use in vaccines has sharply declined since 2001, following the FDA Modernization Act of 1997, which mandated an evaluation of mercury content in childhood vaccines. However, it is crucial to understand that mercury exists in various forms. Methylmercury, which is found in seafood, can be toxic, while ethylmercury, the form found in thimerosal, is processed and eliminated by the body much more quickly. Fiscus noted that despite thimerosal's removal from most childhood vaccines in 2001, no evidence has indicated that it caused harm.
Concerns regarding a potential link between vaccines and autism were fueled by a now-retracted 1998 study by Dr. Andrew Wakefield. This study, which focused on the MMR vaccine, has been thoroughly discredited, and Wakefield lost his medical license. The National Academies of Sciences Engineering and Medicine published a report in 2004 confirming that thimerosal is not associated with autism. Subsequent research in 2006, 2007, 2009, and 2010 further supported the conclusion that thimerosal does not contribute to autism or neurophysiological issues. Goodman pointed out that despite thimerosal's removal, autism rates have continued to rise over the last two decades, indicating no correlation with childhood vaccines.
Most individuals may not notice if thimerosal is effectively banned from flu shots, as single-dose syringes are commonly used for administration. However, a complete ban could pose logistical challenges, particularly for health centers that typically begin ordering flu vaccines in February for the upcoming season. Manufacturers may need to adjust their production processes. Sanofi, which produces the multidose Fluzone vaccine as well as single-dose options, indicated that only a small fraction of its doses contain thimerosal. They are prepared to meet demand regardless of the CDC's decisions.
Similarly, Seqirus, another manufacturer of multidose flu vaccines, assured that thimerosal constitutes a minor part of its overall supply and expressed commitment to transitioning fully to single-dose syringes without significant disruptions.
Dr. Fiscus expressed concern that the ACIP's decision to recommend only thimerosal-free flu shots reflects a willingness to prioritize public perception over established scientific evidence. "Is this going to become the new standard?" she questioned. Goodman echoed these sentiments, cautioning that such moves could ultimately undermine public confidence in vaccines in both the United States and globally.
