for the official statement from the CDC
What are the risks from getting a flu shot?
The viruses in the flu shot are killed (inactivated), so you cannot get the flu from a flu shot. The risk of a flu shot causing serious harm or death is extremely small. However, a vaccine, like any medicine, may rarely cause serious problems, such as severe allergic reactions. Almost all people who get influenza vaccine have no serious problems from it.
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What are the side effects that could occur?
Soreness, redness, or swelling where the shot was given
Fever (low grade)
Aches
The intradermal flu shot may cause other additional mild side effects including:
Toughness and itching where the shot was given
If these problems occur, they begin soon after the shot and usually last one to two days.
Can severe problems occur?
Life-threatening allergic reactions are very rare. Signs of serious allergic reaction can include breathing problems, hoarseness or wheezing, hives, paleness, weakness, a fast heartbeat, or dizziness. If they do occur, it is within a few minutes to a few hours after the shot. These reactions are more likely to occur among persons with a severe allergy to eggs, because the viruses used in the influenza vaccine are grown in hens’ eggs. People who have had a severe reaction to eggs or a flu shot in the past should not get a flu shot before seeing a physician.
Guillain-Barré syndrome: Normally, about one person per 100,000 people per year will develop Guillain-Barré syndrome (GBS), an illness characterized by fever, nerve damage, and muscle weakness. In 1976, vaccination with the swine flu vaccine was associated with getting GBS. Several studies have been done to evaluate if other flu vaccines since 1976 were associated with GBS. Only one of the studies showed an association. That study suggested that one person out of 1 million vaccinated persons may be at risk of GBS associated with the vaccine.
What should I do if I have had a serious reaction to seasonal influenza vaccine?
Call a doctor, or get to a doctor right away.
Tell your doctor what happened, the date and time it happened, and when you got the flu shot.
Ask your doctor, nurse, or health department to file a Vaccine Adverse Event Reporting System (VAERS) form, or call VAERS at 1-800-822-7967.
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Can the flu shot give me the flu?
No, a flu shot cannot cause flu illness. The viruses contained in flu shots are inactivated (killed), which means they cannot cause infection. Flu vaccine manufacturers kill the viruses used in the flu shot during the process of making vaccine, and batches of flu vaccine are tested to make sure they are safe. In randomized, blinded studies, where some people got flu shots and others got saltwater shots, the only differences in symptoms was increased soreness in the arm and redness at the injection site among people who got the flu shot. There were no differences in terms of body aches, fever, cough, runny nose or sore throat.
More information about these studies is available at:
Carolyn Bridges et al. (2000). Effectiveness and cost-benefit of influenza vaccination of healthy working adults: A randomized controlled trial. JAMA. 284(13):1655—1663.
Kristin Nichol et al. (1995). The effectiveness of vaccination against influenza in healthy working adults. New England Journal of Medicine. 333(14): 889-893.
http://www.cdc.gov/flu/about/qa/flushot.htm