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Cold and Flu Center

Flu Shot Side Effects, Including Fatigue

author image Diane Helentjaris
Diane Helentjaris is a medical writer and past president of the American Medical Women's Association. She received her Bachelor of Arts (humanities) and Medical Doctorate from Michigan State and her master's degree in public health from the University of Michigan. She is currently researching her first book which combines both medicine and history.
Flu Shot Side Effects, Including Fatigue
A doctor injecting a syringe into a man's arm in an exam room. Photo Credit Wavebreakmedia Ltd/Wavebreak Media/Getty Images

If the odds of winning the lottery were 1 in 5, the line for tickets would be long indeed. In fact, as many as 1 of every 5 Americans will get the flu this year, according to the Centers for Disease Control and Prevention. These high odds could be decreased if more Americans took advantage of influenza immunization. The CDC estimates that vaccination can cut flu-related visits to medical facilities by 60 percent. But worries about other odds – particularly the odds of experiencing negative side effects from a flu shot – keep some folks from getting vaccinated even though the benefits generally outweigh the risks.

Most Common Side Effect

Flu shots have been available commercially in the U.S. since the 1940s. Nowadays, there are a variety of these injectable vaccines, all with similar potential side effects. The most frequently reported side effect is soreness at the injection site. The CDC has found that 10 to 64 percent of adults experience some soreness, which typically lasted less than 2 days and did not interfere with daily activities. Two vaccines – one with a stronger virus dose and the other given into the skin instead of the muscle – are more often associated with injection site reactions.

Other Common Side Effects

Adults may faint after receiving their flu shot. Headache, fever, fatigue, malaise, muscle aches and nausea are other mild transient side effects that people may experience after being immunized. Since the injectable flu vaccines are all created using killed influenza virus, it is not possible to catch the flu from a flu shot. However, it takes on average about 2 weeks for the vaccination to become fully effective; since influenza may already be circulating in the community, it is possible to catch the flu before the shot has had time to become protective.

Allergic Reactions

Allergic reactions can occur from sensitivity to a vaccine component. Allergic reactions can be mild to severe, and include anaphylaxis. Anaphylaxis is potentially fatal unless treated. Anaphylaxis can manifest itself with hives or urticaria; wheezing; swelling of the tongue, lips or face; difficulty breathing and loss of consciousness. Anyone with suspected anaphylaxis or other severe allergic reaction requires immediate medical attention. The CDC reports that anaphylaxis is rare, with less than 2 cases reported for every million doses of vaccine given. As chicken eggs are used to make most flu vaccines, persons with egg allergies should take precautions. Recently vaccines have been developed that do not utilize eggs in the manufacturing process.

A Possible Rare Effect

Guillain-Barré syndrome is a very rare autoimmune condition that appears to follow infections with viruses, including influenza and other infectious diseases. Inflammation of the nerves causes progressive muscle weakness and in some cases, paralysis. While research into the risk of developing Guillain-Barré syndrome after a flu shot is ongoing, some precautions are recommended. Persons who have recently been ill with Guillain-Barré syndrome should discuss their situation with a health care provider before being immunized. The CDC estimates that 1 case of Guillain-Barré syndrome may occur for every million doses of flu vaccine given. Persons who suspect they have Guillain-Barré syndrome should seek prompt medical evaluation.

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