In an interview with Pharmacy Times®, Adam James, PharmD, and Chris Altman, PharmD, explain the history and function of thimerosal as a preservative in multi-dose influenza (flu) vaccines, clarify its safety profile, and examine why public concern over the compound has re-emerged. James and Altman describe how social media and shifting CDC/Advisory Committee on Immunization Practices (ACIP) discussions have fueled vaccine hesitancy, despite longstanding evidence of thimerosal’s safety and limited use in today’s vaccine supply. The pharmacists also break down the June 2025 ACIP recommendation, which advises against using thimerosal-containing influenza vaccines for the upcoming 2025–2026 flu season.
Pharmacy Times: Could you discuss what exactly thimerosal is and its role in vaccines?
Chris Altman, PharmD: It’s great to have this conversation about thimerosal—specifically what it is—and we’re hoping to address some key issues and break it down from a community pharmacist’s perspective. So I think it’s great to start with a simple question: what is thimerosal? Simply put, it’s a preservative. But Adam, do you want to dive in a little more on how it’s used and why it’s used?
Adam James, PharmD: Yeah, absolutely, Chris. As Chris mentioned, thimerosal is a preservative used in vaccines. It’s only used in multi-dose vaccine vials. What a preservative is intended to do is prevent contamination from microbes like bacteria and fungi. We’ve used thimerosal in vaccines since the 1930s—nearly 100 years. If we didn’t use it in multi-use vials, where we’re sticking a needle in the vial multiple times, the product could be exposed to contamination and no longer be considered safe to administer. One thing to mention—and this is the reason it’s a hot-button topic—is that it is a mercury-containing preservative. But we’ll get into the differences in mercury-containing products later.
Key Takeaways
- Thimerosal is a mercury-containing preservative used only in multi-dose flu vials, with a long-standing safety record dating back to the 1930s.
- The ACIP has recommended against using thimerosal-containing flu vaccines for all patient groups during the 2025–2026 season, reflecting a precautionary stance rather than new safety concerns.
- Social media misinformation and heightened public attention to vaccine ingredients have reignited debates, underscoring the need for pharmacists to engage patients with empathy and clear education.
Pharmacy Times: Why has the debate over thimerosal become inflamed in recent months?
Altman: Thimerosal has come up more and more recently. I think there are a couple of things driving that. Number one, the way patients consume information has changed. When we have hot-button topics, they can rise quickly and spark conversations rapidly. Information—good or bad—spreads very fast through social media, blogs, podcasts, and other platforms. With some of the changes and shifts at the CDC and ACIP, thimerosal became a topic of conversation again. The ACIP held a meeting in June and discussed thimerosal-containing flu vaccines, which kicked off a lot of conversation. I also think it’s partly due to our patients being more attuned to vaccine safety profiles—what they’re choosing to give and what that schedule looks like. Adam, I don’t know if you’ve seen more of that from pharmacists or patients you’ve worked with.
James: Yeah. One thing I’ve heard in particular is the increased social media attention around vaccine safety and hesitancy. Some of it is valid, some less so—it really depends on how medically informed someone is. A lot of social posts are very convincing and fear-inducing. If you don’t have a medical background or the baseline knowledge we have as pharmacists and immunologists, it’s easy to get misled. What’s important to understand about social posts is that anyone can post anything with very little limitation or consequence. So, as pharmacists, we need to be aware that our patients are seeing this information—and it’s definitely driving the conversation. We're seeing it pop up across all kinds of platforms.
Pharmacy Times: What does ACIP’s recent recommendation regarding thimerosal in influenza vaccines mean for patients and pharmacists?
Altman: At the ACIP meeting in June, there was a conversation led around the safety and efficacy of thimerosal. I had the pleasure of watching the meeting. The presentation discussed thimerosal’s potential toxicity and whether it’s an effective agent to prevent bacterial contamination. The speaker laid out reasons against using thimerosal, but—as we mentioned earlier—it’s been used since the 1930s and has robust safety data. We’ve already seen steps taken by the CDC and FDA to reduce thimerosal exposure. There’s not a lot of thimerosal in vaccines today. The ACIP broke the conversation down by age groups—should thimerosal-containing flu vaccines still be used in children, pregnant women, and adults? Ultimately, the ACIP voted to recommend against using thimerosal-containing flu vaccines for any patient during the 2025–2026 flu season. It was an interesting conversation to watch. Some of the conclusions seemed to leap ahead of the presented data, showing how fast this issue bubbled up and sparked debate at a national level.
James: And just to clarify, this recommendation isn’t fully new. Since the early 2000s, there’s been a significant push to remove thimerosal from all childhood vaccines—which has been successful. Now, thimerosal is only present in a few multi-dose flu products. Very few vaccines being administered today contain it.
