Flu shots, more formally known as influenza vaccines, are made from inactivated viruses. The flu shot for the 2010-2011 flu season contains a "killed" form of three strains of influenza virus, including the 2009 H1N1 strain of influenza A, a strain of influenza B and the H3N2 virus. According to the Centers for Disease Control and Prevention, because these injections contain no live viruses, there's no chance that a flu shot will cause an influenza infection. However, flu shots can cause both minor and more severe reactions.
Injection Site
According to the CDC, some people experience redness, soreness or swelling around the site where the vaccine was injected. The symptoms are usually mild and last only a day or two after the immunization.
Respiratory Symptoms
The CDC reports that the flu shot can also cause mild hoarseness, cough or itchy red eyes that begin shortly after the injection and last one to two days. It's important to distinguish these mild reactions from a life-threatening allergic response known as anaphylaxis, which begins from a few minutes to a few hours after the flu shot. According to Flu.gov, the allergic response also involves cough and hoarseness, but it quickly progresses to include breathing difficulties, swelling around the eyes and lips, hives, weakness, and dizziness. It's important to obtain medical attention immediately if these symptoms occur.
Fainting
Flu.gov differentiates between a simple fainting episode that occurs when someone receives an injection and more serious confusion or behavioral changes that indicate a severe reaction to the flu vaccine. An individual who has fainted will recover after lying down briefly, but significant alterations in mental status or behavior should be reported to a physician immediately.
Fever
While a flu shot often causes a mild fever, the person's temperature should drop within a day or two. If the fever persists or worsens, it's time to notify the doctor. One specific brand of inactivated flu vaccine known as Afluria has been linked to fevers and febrile seizures in children, so it should not be used in youngsters under the age of 8 years, advises the CDC.
Guillain-Barre Syndrome
In 1976, an inactivated influenza vaccine for a form of the swine flu was linked with a severe paralytic illness called Guillain-Barre syndrome. While modern flu shots have not been associated with this complication, any muscle weakness or loss of function should be reported immediately to a health care provider.
