Flu Shot: The Vaccine and Its Side Effects

Medically Reviewed by Minesh Khatri, MD on November 13, 2021
2 min read

The flu vaccine is the best way to protect yourself from the influenza virus. You should get one every year, unless you have a medical reason not to.

Flu season can begin as early as October and last as late as May. It’s best to get the vaccine as soon as it becomes available each fall. But you can still get vaccinated in January or later. The flu shot becomes effective about 2 weeks after you get it.

No. The viruses in the flu shot are dead. Even the nasal spray, which has a weak version of the flu virus, cannot give you the flu.

Most people have no problems from the vaccine.

If you get the flu shot, you might have a mild fever and feel tired or achy afterward. Some people also have soreness, redness, or swelling where they got their shot. These problems aren’t serious and don’t last long.

Serious side effects are rare. If they do happen, it's within a few minutes to a few hours after you get the shot. Call your doctor right away if you have trouble breathing, hives, feel weak or dizzy, or have a fast heartbeat afterward.

If you get the nasal spray, you might have side effects like a runny nose, headache, cough, and sore throat. These are milder and shorter than the flu.

Some people should make sure it’s OK to get vaccinated. Ask your doctor or pharmacist first if:

  • You’ve had a life-threatening allergic reaction to a flu shot in the past.
  • You’ve had Guillain-Barre syndrome that happened after you got the flu vaccine. That’s a disorder in which the body's immune system attacks part of the nervous system.
  • You’re very ill. If you have a mild illness, it's OK to get vaccinated. Otherwise, talk to your doctor or pharmacist first.

Show Sources


CDC: "Estimating Seasonal Influenza-Associated Deaths in the United States: CDC Study Confirms Variability of Flu," “Key Facts About Seasonal Flu Vaccine.”


American Lung Association.

National Institute of Allergy and Infectious Diseases: "Flu (Influenza): Prevention."

National Jewish Medical and Research Center: "Is it better to use a flu inhaler or get a flu shot?"

American Lung Association: "Cold and Flu Guidelines: Influenza."

FDA: "Influenza: Vaccination Still the Best Protection."

MedlinePlus: "Flu," "Common Cold."


Flu Near You website.

News release, American College of Allergy, Asthma and Immunology.

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