Livestrong.com
The flu shot is an inactivated virus vaccine usually given in the arm for adults and children and the thigh for infants. According to the Centers for Disease Control and Prevention (CDC), getting an annual flu shot can reduce your risk of getting influenza by 70 to 90 percent. However, you should know more about the risks and dangers of flu shots before you decide to get vaccinated.
Injection Site Reaction
Injection site reaction is one of the most common side effects of the flu shot. In clinical trials for the FDA-approved vaccine Afluria, the most common symptoms were tenderness (60 percent), pain (40 percent), redness (16 percent), swelling (9 percent) and bruising (5 percent). One percent of people reported symptoms that were severe enough to interfere with activities. In rare cases, an injection site reaction is a harbinger of more severe complications such as abscess–a localized collection of pus beneath the skin–or cellulitis–an infection of the skin. According to MedlinePlus, both abscess and cellulitis are treated with antibiotics. Abscess may also require surgical drainage.
Allergic Reactions
Allergic reactions are occasionally reported in people who receive the flu shot. In fact, according to the prescribing information, certain kinds of allergies are the only absolute contraindications to receiving a flu shot. Allergic reactions range in severity from hives (wheals), itching and rashes to more severe complications such as angioedema and anaphylaxis. The main symptom of angioedema is sudden swelling that, unlike the swelling associated with hives, occurs below the skin. Swelling can be so severe that it interferes with breathing. Angioedema can also be a precursor to the most feared allergic reaction: anaphylaxis.
Anaphylaxis, according to MedlinePlus, is a severe, life-threatening allergic reaction involving the entire body. Initial symptoms may include abdominal pain/cramping, breathing problems, mental confusion, fainting, light-headedness, dizziness, hives or itchiness, heart palpitations and a feeling of doom. Without immediate medical attention, cardiac or respiratory failure and death often ensue. Although the danger of allergic reaction is real, the risk is low. In the Dec. 4 2009 “Morbidity and Mortality Weekly Report,” the CDC announced that only 16 “serious allergic reactions” and 18 confirmed cases of anaphylaxis had been reported out of approximately 145.1 million administered doses of seasonal and H1N1 flu vaccine.
Guillian-Barre Syndrome
Guillain-Barre (pronounced ghee-YAN bah-RAY) syndrome (GBS) is a disease in which a person makes antibodies against her own nerves. The hallmark of GBS is ascending paralysis that begins in the legs and spreads to the trunk of the body and arms. According to the CDC, GBS can last for weeks or months and sometimes causes 
death. About 1 in 1 million people in the United States develops GBS each year. In 1976, among people who were vaccinated for the swine flu, 1 in 100,000 developed GBS. Between 1992 and 1994, the risk of developing GBS among people vaccinated for seasonal flu was 1.7 times that of people who were not vaccinated; however this figure is controversial because it amounts to less than one extra case per million people vaccinated.
Nervous System Disorders
In addition to GBS, which receives its own warning, the flu shot has also been linked to other kinds of nervous system disorders. Optic Neuritis (ON) consists of eye pain and vision loss that affect one or both eyes. According to a 2002 article in the journal “Current Opinions in Neurology,” cases of ON following vaccination with inactivated-virus vaccine were reported in 1996, 1997, 1998 and 2004. The patient in 1996 experienced a resolution of symptoms, only to have them recur with re-vaccination in 1997.
Acute Disseminated Encephalomyelitis (ADE), according to a 2008 article in the “Journal of Clinical Neuroscience” has been described in adults and children who received several vaccines, including the flu shot. The disease presents with the sudden onset of fever, muscle weakness, increased or decreased reflexes, vision changes, impaired balance, loss of ability to read or write, inability to remember previously known words and other neurological symptoms. In order to be considered vaccine-related, the symptoms must begin within three months of receiving the flu shot. Patients usually recover, however the process may take months or years.
Vascular Disorders
The flu shot has been linked to a kind of inflammation of the arteries known variously as temporal arteritis, giant cell arteritis and cranial nerve arteritis. According to a 2002 article in “Current Opinions in Neurology,” the first case was described in 2000. Symptoms include new, severe headaches; visual disturbances; jaw and tongue pain associated with swallowing; and tenderness in the visible arteries across the forehead. In most cases, symptoms resolved with prolonged (up to three years) steroid treatment.