Is a Preservative-Free Flu Shot Safer?

Learn about the safety of thimerosal, a preservative used in some vaccines

A preservative-free flu shot is a type of flu vaccine that does not contain the antimicrobial agent known as thimerosal. Thimerosal, a mercury-based compound, is an ingredient in some vaccines because it prevents the growth of bacteria, fungus, or other microorganisms that might contaminate a vial when a needle is inserted.

Some people seek out preservative-free vaccinations because they've heard that vaccine ingredients like thimerosal can cause autism, a claim that clinical researchers have repeatedly been debunked.

According to the Centers for Disease Control and Prevention (CDC), the side effects of flu shots containing thimerosal are ultimately the same as those that are preservative-free.

How Safe Is Thimerosal?

Thimerosal is used in multi-dose vials of the flu shot for delivery to multiple people. This helps ensure the purity of the vaccine from one person to the next.

Prior to the introduction of thimerosal, there were cases where a multi-dose vial was contaminated with bacteria between uses, leading to illness and even death. Preservatives became a requirement for multi-dose vaccines in 1968, although they were in use well before then.

Despite the proven safety of thimerosal in multi-dose vaccines, there were many who remained unconvinced.

In 1998, the safety of vaccines was cast into further doubt when a group of scientists in Great Britain published a study suggesting that the measles, mumps, and rubella (MMR) vaccine caused children to develop autism.

Although the study was published in one of the leading medical journals, The Lancet, it soon drew the ire of scientists who noted numerous flaws in the study model. By 2010, The Lancet issued a full retraction of the article and its findings.

Nevertheless, the study set off a firestorm of misperceptions that led many parents to abandon vaccines, as well as the preservatives perceived to be harmful, altogether.

As a result of the "public's perception of risk," as described by the World Health Organization (WHO), thimerosal was discontinued for use in most multi-dose vaccines in 2001. All other single-dose vials in the United States are preservative-free, except for Fluvirin, which contains only a trace amount from the use of thimerosal in its manufacturing process.

To date, there has been no evidence that thimerosal can or has ever caused autism in children. According to CDC, "the number of children diagnosed with autism has not gone down since thimerosal was removed from vaccines."

How Is Flu Vaccine Safety Evaluated?

The CDC, Food and Drug Administration (FDA), National Institutes of Health (NIH), American Academy of Pediatrics (AAP), Advisory Committee on Immunization Practices (ACIP), and National Academy of Sciences' Institute of Medicine (IOM) have all declared that the use of thimerosal in vaccines is safe and effective.

The safety profile of traditional and preservative-free flu shots is considered equal. Possible side effects of both thimerosal-containing vaccines and preservative-free vaccines include:

  • Pain and swelling at the injection site
  • Low-grade fever
  • Fatigue

Side effects are typically mild and resolve within a day or two.

However, if you develop wheezing, difficulty breathing, vomiting, rapid heartbeat, loss of consciousness, or swollen lips, tongue, or throat, call 911 or go to your nearest emergency room. These could be the signs of a rare and potentially life-threatening allergic reaction known as anaphylaxis.

How the FDA Evaluates Vaccine Safety

The FDA is tasked with ensuring the safety of vaccines. Before any vaccine is approved, it is tested by its manufacturer with three stages (or phases) of clinical trials, the last of which involves hundreds or thousands of participants. After the vaccine is licensed, it is monitored for any adverse events through the nationwide Vaccine Adverse Event Reporting System (VAERS), and other monitoring systems. If the CDC reviews the VAERS data and decides it is unsafe, the vaccine will be withdrawn from the market.

When Do I Need a Flu Shot?

According to the CDC, you should get your flu shot (whether traditional or preservative-free) before the end of October. Children under nine receiving the vaccine for the first time would need a second shot at least four weeks after the first.

It is important to note that preservative-free flu vaccines contain the same four strains of killed influenza virus as other injectable so-called quadrivalent flu shots. Preservative-free flu shots are also delivered at the same time and in the same dose as the traditional flu vaccines.

Flu shots, whether traditional or preservative-free, are contraindicated for use in:

  • Children younger than six months
  • People with severe, life-threatening allergies to the flu vaccine or any ingredient in the vaccine

Approximately 93% of the vaccine supply produced for the 2022 to 2023 flu season is preservative-free.

Do I Need a Preservative-Free Flu Shot?

Unless flu vaccination is contraindicated for you, there is no safety-related reason to avoid getting a traditional flu shot.

With that said, if you would prefer to avoid thimerosal, ask your healthcare provider if they have a preservative-free flu vaccine available. Alternatively, you can ask your healthcare provider if FluMist nasal spray vaccine is an option for you.

While the FluMist vaccine is preservative-free, it contains a live weakened virus and should not be used in people with a compromised immune system, children under two years, adults 50 and over, pregnant women, people with a severe, life-threatening reaction to flu shots or any of their ingredients, and people with other medical conditions.

Be aware that most clinics and pharmacies will only have one or two types of vaccine on hand (typically the traditional multi-dose vial and FluMist). A special order would need to be placed for you if you would like something different.

Because it takes about two weeks for the body to produce enough defensive antibodies, the vaccination should be done early in the season to avoid infection.

A Word From Verywell

Although there is no evidence that thimerosal causes harm when used in vaccines, there are other options to choose from if you'd rather avoid it.

Ultimately, what's most important is that you get vaccinated against the flu, especially if you are at high risk for flu-related complications. And, despite what some people might tell you, the flu shot does not cause the flu.

Frequently Asked Questions

  • Which brand of flu vaccine is preservative-free?

    Most single-dose flu vaccine vials, pre-filled syringes of flu shots, and the nasal spray flu vaccine (FluMist) do not contain a preservative because they are intended to be used once. If unsure, ask your pharmacist or healthcare provider giving you the vaccine.

  • What does thimerosal do to the body?

    No harmful effects have been reported from thimerosal at doses used in vaccines, except for minor reactions such as swelling or redness at the injection site lasting for one to two days. It is very unlikely you will have an allergic reaction to thimerosal.

  • Does the preservative-free flu shot contain formaldehyde?

    No, but some multi-dose flu vaccines may contain trace amounts (0.1 milligrams or less). It is important to note that formaldehyde is essential in metabolism and the synthesis of the building blocks of protein. The amount of formaldehyde in flu vaccines is much smaller than the amount naturally occurring in the body. Even in infants, an average of 1.0 milligrams of formaldehyde is normally circulating in the blood at any one time.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.