Is a Preservative-Free Flu Vaccine Safer?

The preservative-free flu vaccine is a type of flu vaccine that does not contain thimerosal. The mercury-based compound is added to some vaccines to prevent the growth of bacteria, fungus, or other microorganisms that might contaminate a vial when a needle is inserted. Some seek out preservative-free vaccines because they're concerned that vaccine preservatives cause autism, but that claim is unsupported by reputable scientific evidence. Side effects of traditional flu vaccines are the same as those for preservative-free options.

Why There Are Preservative-Free Vaccines

Thimerosal is used in multi-dose vials of the flu vaccine for delivery to multiple people to ensure the purity of the vaccine from use to use. Prior to using this preservative, 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 before that date.

This, of course, begs the question as to why there is a preservative-free flu vaccine option altogether.

Back in 1998, a doctor in Great Britain published a small study claiming that the measles, mumps, and rubella (MMR) caused children to develop autism.  The study was incredibly flawed both in design and controls. Subsequent research failed to show any link between the MMR vaccine and autism. The journal where the study appeared, The Lancet, issued a full retraction in 2010.

Nevertheless, the study set off a runaway firestorm of misperceptions and falsehoods that led many parents to abandon vaccines altogether, as well as any ingredient contained in them. Thimerosal was one such ingredient that was often blamed.

Due to solely to unwarranted public fear and an overabundance of caution, thimerosal has not been used in any vaccine other than the multi-dose flu vaccine since 2001.

Preservative-free flu vaccines come in a single-dose vial, as they do not contain ingredients to help prevent microorganism contamination.

Safety and Side Effects

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

The safety profile of the traditional and preservative-free flu vaccines are considered equal. Like all injected flu vaccines, side effects of the preservative-free flu vaccine 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 severe pain, increased swelling, nausea, vomiting, rash, rapid heart rate, or difficulty breathing, call 911 or go to your nearest emergency room.

Vaccination and Contraindications

According to the CDC, you should get your flu shot (whether traditional or preservative-free) before the end of October. Children under age 9 receiving the vaccine the first time would need a second shot four weeks after the first.

Approximately 85% of the vaccine supply produced for the 2019 to 2020 flu season was thimerosal-free (i.e., preservative-free). For this particular season, only multi-dose vials contains thimerosal.

The formulation contains the same four strains of killed influenza virus as other injectable quadrivalent flu vaccines. Preservative-free flu vaccines are also delivered at the same time and the same dose as the traditional flu vaccines.

Those who should not get the preservative-free flu vaccine include:

• Anyone with a severe egg allergy
• Anyone with a history Guillain-Barré syndrome
• Anyone with an allergy to any of the ingredients in the flu vaccine
• Anyone with a significant fever at the time of vaccination
• Infants under 6 months old

Considerations

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

That said, if you would prefer to avoid thimerosal anyway, ask your healthcare provider if they have a preservative-free flu vaccine available. Most immunization clinics and pharmacies will only have one or two types on-hand (typically the traditional multi-dose vial and the FluMist nasal spray) and would have to place a special order for you.

Because it takes two weeks for the body to produce defensive antibodies after the shot, this would need to be done early in the season to avoid infection. With the changes made for the 2019 to 2020 season, however, it shouldn't be too difficult to obtain the thimerosal-free vaccine. Alternatively, you can ask your doctor if FluMist is a reasonable option.

In future flu seasons, it should remain likely that preservative-free single-dose vaccines are available. The only wrinkle could be that a vaccine against a newly-circulating strain is rushed into production and there is a lag in production or availability of single-dose vials as opposed to multi-dose vials.

A Word From Verywell

Although there has been no evidence that thimerosal causes harm when used in vaccines, there are other options today that do not contain this ingredient if you'd rather avoid it. What's most important is that you get vaccinated against the flu, especially if you are at high risk for flu-related complications.