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Flu Shot: The Vaccine and Its Side Effects

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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.

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The Flu and You: Your Urgent Response Guide

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Can the Vaccine Give Me the Flu?

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.

What Are the Possible Side Effects?

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 after you get vaccinated. 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.

Should I Talk to My Doctor Before I Get Vaccinated?

Some people should make sure it’s OK to take the vaccine. Ask your doctor first if:

  • You’ve had an 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 first.

WebMD Medical Reference

Reviewed by Roy Benaroch, MD on July 08, 2015
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