Side Effects of the Influenza Vaccine

The influenza vaccine is the best protection against seasonal flu, which is why millions of people get the flu shot each year. While very few serious side effects have been reported, people have been known to experience mild side effects to the flu shot or nasal spray flu vaccine (FluMist).

Common Side Effects

The annual influenza vaccine offers safe and effective protection against the seasonal flu. The side effects are typically mild and due to an immune response that effectively boosts your immunity to the virus.

Side effects common to both the flu shot and FluMist nasal spray include:

• Low-grade fever
• Headache
• Muscle aches
• Fatigue
• Malaise

There are also side effects specific to the flu shot and FluMist nasal spray. For example, cough can occur after the administration of FluMist.

Flu Shot Side Effects

Flu shots involve inactivated viruses that have been killed and are not infectious. Side effects can include:

• Pain at the injection site
• Redness and swelling at the injection site

FluMist Side Effects

The FluMist nasal spray is a live attenuated influenza vaccine (LAIV) made with live viruses that have been weakened and are unable to cause influenza illness. The attenuated viruses can only multiply at cooler temperatures, like those in the nose, and cannot survive at body temperature.

Side effects can include:

• Runny nose and nasal congestion
• Wheezing (typically in children)
• Nausea or vomiting
• Sore throat

Despite a common myth that the flu vaccine can give you the flu, neither the flu shot nor the flu nasal spray will infect you with influenza.

Rare Side Effects

Although rare, serious allergic reactions including anaphylaxis may occur following vaccination. Signs of anaphylaxis include:

• Rash or hives
• Wheezing
• Shortness of breath
• Rapid heartbeat
• Swollen tongue, lips, or throat
• Dizziness or lightheadedness
• Confusion
• A feeling of impending doom

Anaphylaxis is considered a medical emergency and, if left untreated, can lead to unconsciousness, shock, coma, heart or respiratory failure, and death.

Anaphylaxis typically occurs within five to 30 minutes of coming into contact with an allergy-causing substance (allergen), although some cases can take more than an hour.

Anyone who has experienced a severe, life-threatening allergic reaction to a flu vaccine in the past should not receive one in the future. Of note, people who have had a mild allergic reaction (such as hives the next day) can and should continue to receive the annual flu vaccine.

Egg Allergies

For many years, flu shots were avoided by people with egg allergies because the vaccine was initially grown in chicken eggs, posing a potential risk for a reaction.

New recombinant flu vaccines—such as Flublok quadrivalent (for adults 18 and older) and Flucelvax quadrivalent (for people 4 years and older)—are manufactured without eggs, making them safe for people with egg allergies.

While you should tell your healthcare provider if you have an egg allergy before getting the flu shot, know that this should not prevent you from getting the vaccination.

The risk of an allergic response to any flu vaccine is extremely low—including those that are egg-based. As such, the CDC recommends the vaccine even for people who have a history of egg allergies or have experienced mild hives following vaccination.

Those with a history of severe allergic reactions to eggs should get the flu shot from their healthcare provider, who can spot signs of a reaction and manage the symptoms quickly.

Vaccines and Autism

Rumors have long circulated suggesting that the flu vaccine may cause autism. It has been proposed preservatives such as thimerosal are responsible for this effect.

Thimerosal, an ethyl mercury-based preservative, was once considered a possible trigger for autism. Research has shown that this is not the case. According to the CDC, thimerosal has a long history of safety with no evidence of harm caused by the low doses used in vaccines.

If you are concerned about preservatives in the flu vaccine, talk to your healthcare provider about preservative-free options. Most single-dose vials and prefilled syringes do not contain a preservative because the products are used immediately and not shared. The same applies to the FluMist nasal vaccine, which is also preservative-free.