The flu shot is the best protection against seasonal flu. This is why millions of people get the influenza vaccine 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).

This article discusses both mild and common side effects and their symptoms. It also explains some of the myths about the vaccine, and special situations for people with a history of allergic reactions.

Common Side Effects

An annual flu shot offers safe and effective protection against the seasonal flu. The side effects are typically mild. They are caused by the immune response that 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
  • General fatigue (malaise)

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

Flu Shot Side Effects

Flu shots are produced by using inactive viruses. This means they have been killed and are not infectious. Side effects from the shot can include pain, redness, and swelling at the injection site.

FluMist Side Effects

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

Side effects of the nasal spray can include:

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

There is a common misconception that the flu vaccine can give you the flu. This is not true, and neither the flu shot nor the flu nasal spray will infect you with influenza.

Rare Side Effects

In very rare cases, people may have serious allergic reactions including anaphylaxis after getting the vaccine.

Recap

The vaccines used to prevent influenza are safe and effective. Many do not contain live virus at all, and even the vaccines that are made with a weaker form of live virus cannot cause the flu. Most people will experience mild side effects, or even none at all. That’s true of both the commonly used flu shots and the FluMist nasal spray.

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 a medical emergency. If left untreated, it can lead to loss of consciousness, shock, coma, heart or respiratory failure, and death.

This severe reaction typically occurs within five to 30 minutes of coming into contact with an allergy-causing substance (allergen). In some cases, symptoms may appear more than an hour later.

Anyone who has experienced a severe, life-threatening allergic reaction to a flu vaccine in the past should not get the vaccine again 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.

A study from the Centers for Disease Control and Prevention (CDC) found the estimated rate of anaphylaxis after all vaccines is 1.31 cases per one million doses. The rate was 1.5 cases per million doses for a typical influenza vaccine.

Egg Allergies

For many years, people with egg allergies avoided flu shots. This is because the vaccine was initially produced using chicken eggs, posing a potential risk for a reaction.

New recombinant flu vaccines are made without eggs, making them safe for people with egg allergies. They include Flublok quadrivalent (for adults 18 and older) and Flucelvax quadrivalent (for people 6 months and older).

You should tell your healthcare provider if you have an egg allergy before getting the flu shot. Still, it’s important to know that this should not prevent you from getting the vaccine.

The risk of an allergic response to any flu vaccine is extremely low. This includes vaccines that are egg-based. As such, the CDC recommends the vaccine even for people who have a history of egg allergies or who have had mild hives after vaccination.

People with a history of severe allergic reactions to eggs should get the flu shot at their healthcare provider’s office. The provider can watch them for signs of a reaction and manage the symptoms quickly.

Vaccines and Autism

For years, there have been rumors to suggest the flu vaccine may cause autism. One of the claims is that preservatives such as thimerosal are a possible trigger for autism.

Flu Shot and Chronic Conditions

It’s true that people with chronic health conditions are at a greater risk of complications from the flu. However, they are at no greater risk for side effects from a flu shot. Flu vaccines have been proven safe in this vulnerable population of people.

Research has shown that this is not the case. According to the CDC, thimerosal has a long history of safety. There is 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 other options. Most single-dose vials and prefilled syringes do not contain a preservative. This is because the products are used right away and not shared. The same applies to the FluMist nasal vaccine, which is also preservative-free.

Summary

An annual flu vaccine is safe and effective for preventing influenza. Many people report mild side effects, like muscle aches or low-grade fever, after getting the shot or nasal spray. These side effects are normal and do not mean that the vaccine gave you the flu. They typically go away in a day or two.

Some people with allergies, notably to eggs, need to be aware of the possibility of a reaction. Recombinant vaccines are an option because they are produced differently. People who have had a severe reaction to the flu vaccine should not receive it again.

A Word From Verywell

Talk to your healthcare provider if you have concerns about flu vaccine safety. That’s especially true if you have heard rumors that the vaccine is unsafe because it may be the cause of autism.

Frequently Asked Questions

  • How long do you feel bad after the flu shot?
  • You may feel soreness for a couple of days in the spot where you were injected with the flu vaccine. If you have side effects like low-grade fever, headache, fatigue, or muscle ache, your symptoms should go away after a day or two.
  • Do you shed the flu virus after getting the vaccine?
  • If you get the live attenuated vaccine, which is in the FluMist nasal spray, it’s possible that you may shed the live virus for up to 11 days after being inoculated. Shedding with this type of vaccine is most common in young children.
  • Does the seasonal flu vaccine affect your heart?
  • In a positive way, yes. Research shows that getting the flu shot can lower your risk of heart attack, heart failure, and other major cardiac events during the following flu season.

You may feel soreness for a couple of days in the spot where you were injected with the flu vaccine. If you have side effects like low-grade fever, headache, fatigue, or muscle ache, your symptoms should go away after a day or two.

If you get the live attenuated vaccine, which is in the FluMist nasal spray, it’s possible that you may shed the live virus for up to 11 days after being inoculated. Shedding with this type of vaccine is most common in young children. 

In a positive way, yes. Research shows that getting the flu shot can lower your risk of heart attack, heart failure, and other major cardiac events during the following flu season.