When it comes to vaccines, it's natural to have questions—especially about ingredients you may not recognize. One name that often comes up is thimerosal. At Crestwood General, we believe that clear, science-based information helps families make confident, informed health decisions. Let's break down what thimerosal is, why it's been used, and what decades of research tell us about its safety.

What Is Thimerosal—and Why Is It in Some Vaccines?

Thimerosal is a preservative that contains ethylmercury. It has been safely used since the 1930s to prevent harmful bacteria and fungi from growing in multi-dose vaccine vials—those small bottles that contain more than one dose.

Why does that matter? Every time a healthcare provider draws a dose from a multi-dose vial, there's a tiny risk of introducing germs. Without a preservative, contamination could lead to serious infections. Thimerosal helps keep vaccines safe and effective from the first dose to the last.

Not All Mercury Is the Same

It's important to understand: not all mercury is created equal.

Methylmercury is the type found in certain fish and the environment. It can build up in the body over time and, at high levels, may cause health concerns.
Ethylmercury, the type in thimerosal, is processed and eliminated by the body much more quickly. It does not accumulate like methylmercury.

Federal guidelines strictly limit methylmercury exposure through food and the environment. Ethylmercury from vaccines has been extensively studied—and found to be safe.

Thimerosal and Childhood Vaccines: What Changed?

In 1999, as a precautionary measure (not because of evidence of harm), U.S. public health agencies and vaccine manufacturers agreed to reduce or eliminate thimerosal from childhood vaccines. By 2001, thimerosal had been removed from or reduced to trace amounts in all routinely recommended vaccines for children ages 6 and under—except for some flu vaccines.

Key facts:
- MMR (measles, mumps, rubella), varicella (chickenpox), IPV (polio), and pneumococcal conjugate vaccines have never contained thimerosal.
- Today, flu vaccines are available in both thimerosal-containing (multi-dose vials) and thimerosal-free (single-dose) options. If you prefer a thimerosal-free flu shot, just ask your provider.
- For a complete, up-to-date list of vaccine ingredients, visit the FDA's Thimerosal in Vaccines page: https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/thimerosal-and-vaccines

What Does the Science Say?

No Link to Autism

This is one of the most thoroughly studied questions in vaccine science. Multiple large, well-designed studies—including research by the CDC, the Institute of Medicine (IOM), and international teams—have found no connection between thimerosal in vaccines and autism.

In fact, after thimerosal was removed from nearly all childhood vaccines in the early 2000s, autism diagnosis rates continued to rise. If thimerosal had been a cause, rates would have been expected to drop—not increase.

A landmark 2010 CDC study confirmed: prenatal and infant exposure to thimerosal-containing vaccines does not increase the risk of autism spectrum disorder (ASD). Learn more about vaccine safety research at CDC Vaccine Safety Studies.

No Impact on Development or Learning

Researchers have also looked closely at whether ethylmercury exposure from vaccines affects children's thinking, behavior, or neurological development. Studies tracking children into their school years have consistently found no link between thimerosal and delays in speech, attention, coordination, or other developmental milestones.

A Quick Look at the Evidence Timeline

Science evolves through careful, ongoing review. Here are a few key moments in thimerosal research:

1999: As a precaution, U.S. health agencies recommend reducing thimerosal in infant vaccines—despite no evidence of harm.
2001: Thimerosal is removed from or reduced in all routine childhood vaccines (except some flu shots).
2004: After reviewing more than 200 studies, the Institute of Medicine concludes there is no link between thimerosal-containing vaccines and autism.
2010: A major CDC study reaffirms that thimerosal exposure before and after birth does not increase autism risk.
2025: CDC and FDA continue to monitor vaccine safety through multiple, robust surveillance systems.

Frequently Asked Questions

Is thimerosal safe?
Yes. Thimerosal has been used safely in vaccines for over 90 years. Decades of research support its safety at the levels used in vaccines.

Why was it removed if it's safe?
Its removal was a precautionary step to increase public confidence and reduce overall mercury exposure—not because evidence showed it was harmful.

Can I get a thimerosal-free flu vaccine?
Yes. Single-dose flu shots and the nasal spray flu vaccine do not contain thimerosal. Just ask your healthcare provider.

What are the side effects of thimerosal?
Most people have no reaction. Rarely, someone might experience mild, short-term redness or swelling at the injection site. Allergic reactions are extremely uncommon.

How can I check if a vaccine contains thimerosal?
Ask your doctor or pharmacist, or review the vaccine's package insert, which lists all ingredients. The FDA also maintains a public database of vaccine components.

How Vaccine Safety Is Monitored

Vaccine safety doesn't stop at approval. In the United States, the CDC and FDA work together through multiple, independent systems to monitor vaccines before and after they're licensed. These systems can detect even rare side effects and ensure recommendations stay up to date.

Learn more about how vaccine safety is protected: CDC Vaccine Safety Monitoring.

Your Health, Your Questions, Our Commitment

At Crestwood General, we know that trust is built on transparency. If you have questions about vaccines, ingredients, or your family's immunization plan, we're here to listen, explain, and support you—without judgment.

For more science-based information, explore these trusted government resources:
- CDC Vaccine Safety: https://www.cdc.gov/vaccinesafety/index.html
- FDA Vaccines and Biological Products: https://www.fda.gov/vaccines-blood-biologics
- National Institute of Allergy and Infectious Diseases (NIAID): https://www.niaid.nih.gov/diseases-conditions/vaccine-safety

Vaccines are one of the safest, most effective tools we have to protect our health—and the health of our communities. With clear facts and compassionate care, we can move forward with confidence.