The flu vaccine is designed to activate the immune system against several strains of flu. It must be reformulated each year because of the gradual evolution of the influenza viruses each year. Flu vaccines can come as an injection or in the form of a nasal spray. Although generally safe, they can cause certain side effects.
Localized Reaction
Some forms of the flu vaccine are given as an injection, the Centers for Disease Control and Prevention explains. When patients receive this form of flu vaccine, which is also known as the inactivated flu vaccine, there is often a localized immune reaction at the sight of the injection. This can result in swelling, redness and soreness at the injection site. The severity of these side effects varies from patient to patient and symptoms generally subside within a few days.
Systemic
Patients who receive a flu shot may also experience some systemic side effects, MedlinePlus explains. These include a low fever and persistent aching of muscles throughout the body. These side effects typically only last a few days.
Flu-Like Symptoms
Flu vaccines can also be administered in the form of a nasal spray, FluMist explains. This type of vaccine uses what is known as an attenuated flu virus for the vaccination, which means that the vaccine contains a form of the flu virus that has been genetically modified to not cause serious infections. Nonetheless, this form of flu vaccine can cause some minor side effects that resemble flu symptoms, such as a runny or congested nose, a sore throat and a fever. Patients may also develop a cough due to mucus from the nose draining down the throat and into the lungs.
Allergic
In some cases patients may have an allergic reaction to the flu vaccine. Often this allergy is not triggered by the active components of the vaccine itself but by other compounds, such as proteins from eggs, which are needed to package and preserve the vaccine. Minor allergic reactions can result in a rash or hives appearing on the body. Other patients may develop swelling of their face, lips, tongue and fingers. Rarely a condition known as anaphylaxis will develop, in which the throat begins to swell and close, restricting swallowing and breathing. These side effects usually occur within the first few minutes of receiving the vaccine, MedlinePlus explains.
Guillain-Barre
Adults who are vaccinated against influenza have an increased risk of developing Guillain-Barre syndrome. This condition is caused by the immune system attacking the lining of the nerves, resulting in neuropathic pain, tingling and muscle weakness or paralysis. It is not known what causes Guillain-Barre to occur, but it sometimes is triggered by an infection. Patients who have ever developed Guillain-Barre after a vaccination should not receive the flu vaccine.
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