Know common side effects and how to soothe them

The flu is caused by influenza viruses that infect the nose, throat, and lungs. These viruses spread when people with the flu cough, sneeze, or talk, which sends droplets of the virus into the air, infecting others who breathe in the germs.

The most effective way to prevent the flu — and complications from the flu — is to get your the flu vaccine.  Most children receive the flu shot every year as the Centers for Disease Control and Prevention recommends. But some parents have questions or concerns about the flu vaccine.

Should I Be Concerned About Ingredients in A Flu Shot?

A recent poll by the C.S. Mott Children’s Hospital found that one in three parents do not plan to vaccinate their children against influenza. The biggest reason for not getting their children the flu shot is concern about flu shot dangers or side effects. These worries often come from not knowing enough about the ingredients in the flu vaccine.

It’s reasonable to have questions about the ingredients in vaccines. Fortunately, the ingredients in flu vaccines have undergone testing for safety in thousands of people. Millions of children safely receive the flu vaccine every year, and studies show that risks related to flu shot ingredients are extremely low.

During the last 50 years, very few people have had any serious problems with the flu vaccine. Research overwhelmingly shows that the flu vaccine and the chemicals it contains are safe.

A common preservative called thimerosal is used in some flu vaccines. Thimerosal has been carefully tested, and studies show that it is very safe.

Possible side effects from thimerosal include redness and swelling at the injection site. A tiny percentage of people may have an allergy to thimerosal, although this is extremely rare. Thimerosal does not build up in the body and instead breaks down and exits the body.

Single-dose flu shots do not contain thimerosal. Multidose flu shots do contain thimerosal to ensure no germs can enter the vial with each new needle.

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What’s in A Flu Shot?

The most important ingredient in a flu vaccine is pieces of the inactivated influenza virus. The virus is cut into pieces in the process of making the vaccine, so it cannot cause the flu. However, when it enters the body through the vaccine, the immune system still recognizes broken flu virus pieces as an intruder.

To fight the intruder, the immune system then produces an army of antibodies. Antibodies are proteins designed to attack a specific germ in the body and then wait in case it comes again. If a real, live flu virus enters the body during flu season, the antibodies attack before the virus can cause disease.

If your child receives the live nasal flu vaccine instead of the shot, it contains a different version of the flu virus. The flu virus in the nasal vaccine is still alive, but the manufacturing process weakens it so much that it can’t cause infection. Again, the body’s immune system doesn’t realize it’s harmless and builds antibodies to remain ready to attack.

Other ingredients in a flu shot include:

Egg protein

Many flu vaccines are made using fertilized chicken eggs. They may contain a small amount of egg protein.


Preservatives in a vaccine help prevent other germs, such as bacteria or fungi, from contaminating the vaccine.


Sucrose, sorbitol, and monosodium glutamate (MSG) are used to prevent vaccines from losing potency, especially when exposed to heat and light.

Sucrose is table sugar. Sorbitol is an artificial sweetener found in some foods. MSG is a flavor enhancer found in many processed foods.


Antibiotics like neomycin and gentamicin stop bacteria from contaminating the vaccine.

Polysorbate 80

In vaccines, polysorbate 80 keeps all the vaccine ingredients evenly distributed. You can find this emulsifier in salad dressings, sauces, and Chinese food. The amount in the flu vaccine is very small.


You may remember formaldehyde from biology class as a preservative used for animals in dissection. Formaldehyde is used to sterilize and preserve things. A tiny amount of formaldehyde is used as a preservative in some vaccines to make them shelf-stable while they’re in storage before use. Our bodies produce formaldehyde as a normal byproduct of digestion and metabolism. The amount of formaldehyde in a flu vaccine is much lower than the amount you already have in your body, where it is easily broken down chemically and discharged harmlessly every day.

Why Are Stabilizers Used in Vaccines?

Stabilizers are used to help vaccines maintain their effectiveness during storage. Vaccine stability is important because vaccines must be transported and stored for periods of time.

Compounds called additives, including stabilizer ingredients, ensure that vaccines remain safe and effective during storage. Here are some examples of additive ingredients in flu vaccine:

  • Gelatin is a stabilizer in some vaccines that protects against damage from heat or freeze-drying for storage. Sugars, artificial sweeteners, and MSG also may be used to keep some vaccines stable and effective.
  • Other possible stabilizers include an amino acid called glycine or a protein in blood called albumin.

All stabilizers used in vaccines occur naturally in the body or are substances people encounter in everyday life. There may also be tiny amounts of substances left over from the process of manufacturing the vaccine:

  • Tiny amounts of egg protein might remain in a vaccine after scientists grew the virus in eggs in a lab.

Flu Shot Side Effects

Side effects from the flu vaccine are usually mild. You may experience:

  • Fatigue.
  • Fever.
  • Headache.
  • Redness or tenderness of the skin around the injection site.

If you have any of the following serious side effects from the flu vaccine, call your doctor or find an emergency room immediately:

  • Dizziness.
  • Fast heartbeat.
  • Hives.
  • Swelling of eyes or lips.
  • Trouble breathing.
  • Weakness.

Is the Flu Shot Dangerous or Unsafe?

The flu shot is very safe for people recommended to get it by the CDC. The most common side effects are soreness, redness, or swelling where the injection occurs. Some people may develop a headache, nausea, or a fever from the vaccine, but they typically go away within a few days.

If your child has allergies to gelatin, antibiotics, or another vaccine ingredient, ask your doctor whether your child should get the vaccine. Most antibiotics used in vaccines are not common ones that people have allergies to. Penicillin is not used in making vaccines.

If your child has an egg allergy, they can still safely receive the flu vaccine. There is also one flu vaccine available for children that does not involve growing the virus in eggs. The Flucelvax Quadrivalent flu vaccine is not made with eggs, and people ages 4 and older can get it.

Editor's Note: This article was originally published on , and was last reviewed on .

Estimates of Flu Vaccination Coverage among Children — United States, 2017–18 Flu Season. National Center for Immunization and Respiratory Diseases. Centers for Disease Control and Prevention. Link

Flu Vaccine and People with Egg Allergies. National Center for Immunization and Respiratory Diseases. Centers for Disease Control and Prevention. Link Vaccine Ingredients: Frequently Asked Questions. The American Academy of Pediatrics. Link

National Poll on Children's Health. Flu vaccine for children in the time of COVID. C.S. Mott Children's Hospital. September 28, 2020. Volume 37, Issue 1. Link

Thimerosal and Vaccines. National Center for Emerging and Zoonotic Infectious Diseases. Centers for Disease Control and Prevention. Link

Understanding Thimerosal, Mercury, and Vaccine Safety. Centers for Disease Control and Prevention. Link

US Food and Drug Administration. Common Ingredients in U.S. Licensed Vaccines. Link

Vaccines & Immunizations. National Center for Immunization and Respiratory Diseases. Centers for Disease Control and Prevention. Link

What's in Vaccines? National Center for Immunization and Respiratory Diseases. Centers for Disease Control and Prevention. Link

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