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The most effective method of flu (influenza) prevention is receiving the annual vaccine. Though it is not 100 percent effective, the vaccine prevents the flu in 70 to 90 percent of healthy people under the age of 65, according to the U.S. Centers for Disease Control and Prevention (CDC). Among some high-risk populations, the flu vaccine is able to prevent hospitalization for pneumonia and the flu in 30 to 70 percent of these patients.

The vaccine is a mix of three forms of the flu (typically two strains of influenza A and one of influenza B) that are expected to cause disease in the coming season. The viruses are either killed and made available as an injection or weakened and provided as an inhaled spray. While anyone older than six months may receive the injection, the inhaled spray is only approved for those between 5 and 50 years of age.

The flu vaccine can help nearly everyone reduce their chance of infection with the virus, but it is strongly recommended for certain groups. Together, these groups total more than 200 million people in the United States according to the CDC. Groups for which the flu vaccine is highly recommended include:

• Children between 6 months and 5 years old.
  
  
• Children between 6 months and 18 years old who are under long-term aspirin therapy (e.g., for conditions such as juvenile rheumatoid arthritis or Kawasaki's disease). The use of aspirin in children with viral illnesses is associated with a risk of Reye syndrome, a serious and potentially fatal disease and the flu vaccine may help minimize this risk.
  
  
• Pregnant women.
  
  
• Patients with chronic lung, heart and metabolic conditions or immunodeficiency.
  
  
• Residents of nursing homes or other long-term care facilities.
  
  
• Adults 50 years of age or older.
  
  
• Household members or caregivers for any of the above.
  
  
• Healthcare workers.

The most frequent side effect of injected flu vaccine is soreness at the site where it was administered. Other side effects may include fever, muscle pain, and general discomfort that may last for a day or two. These are most likely to occur in people who have not been vaccinated before.

Severe reactions (e.g., hives, swelling under the skin, difficulty breathing) are also possible, but are rare. The flu vaccine has also been associated with the autoimmune disorder Guillain-Barré syndrome, though it was rare (less than 10 cases per million people vaccinated according to the CDC).

The inhaled vaccine has only recently become available and its use is more limited than the injected form. Due to its route of delivery, it may not be effective with those currently affected with nasal congestion. Side effects of the inhaled vaccine may include nasal congestion, headache, sore throat, fever, vomiting, and abdominal and muscle pain. Because it is a dose of weakened, but live virus, patients may shed the virus for two days or more after vaccination. Its use is not recommended for people who are severely immunocompromised or those who have close contact with immunocompromised patients. It also is not recommended for pregnant women.

Both forms of vaccine are grown in eggs and may produce allergic reactions in patients who are sensitive to egg products.

In most years, both the injected and inhaled types of vaccine are typically available between September and January. During shortages, this time period may vary. Since it takes six to eight weeks for the vaccine to become active, it is recommended that most patients be vaccinated by the beginning of December.

In addition to the vaccines, antiviral medications used to treat the flu may also be used to help prevent infection. The medications must be taken daily and prophylactic, or preventive, use is recommended only for those who cannot safely be vaccinated.

Personal hygiene can also help prevent flu infections. Regular hand washing helps prevent the virus from moving from possibly contaminated surfaces into the patient’s body. In addition, it is recommended that infected patients cover the mouth and nose when coughing or sneezing and properly dispose of tissues.