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Influenza, commonly known as the flu, does not lead to serious illness in most people. However, it can lead to hospitalization and even death in some individuals, so public health authorities in many countries recommend getting vaccinated against the disease yearly.1

The flu vaccine is approved for people older than six months of age. While everyone should get vaccinated, it’s especially important for people who are more likely to suffer complications from the flu, including pregnant women, infants, young children, the elderly and those suffering from chronic conditions like diabetes or asthma.1

The flu vaccine is produced from influenza virus that has been weakened or inactivated. Since there are variations, known as “strains,” in the flu virus from year to year, and place to place, vaccines protect against the three or four strains that research shows will be most common during the upcoming season. These are known as trivalent (three-sided) or quadrivalent (four-sided) vaccines.1

The flu vaccine is given either in the form of an injection or a nasal spray, which some people find more comfortable. Whatever type of vaccine you and your doctor decide is best for you, it’s important to get vaccinated every year for two reasons:2

  1. The antibodies that your body produces to protect you from the virus after vaccination start to decline over time.
  2. Flu vaccines are updated regularly to provide optimal protection from the rapidly changing virus.

You cannot get the flu from either the flu shot or the nasal vaccine. However, you may experience flu-like symptoms after vaccination due to the following:3, 4

  • Other Respiratory Illnesses: You may be infected with another respiratory virus, such as the common cold, which can cause many of the same flu-like symptoms.
  • Two-Week Waiting Period: You may have become infected with the flu shortly before you were vaccinated or during the two-week period that it takes for the body to gain protection after vaccination.
  • Vaccine Side Effects: After receiving the flu vaccine, your body’s immune system produces protective antibodies, which may cause some flu-like symptoms.
  • Different Flu Strains: Since the flu virus changes frequently, you may have caught a strain that is different than the one you were immunized against.
  • Age and Chronic Illness: After vaccination, some older adults and people with chronic conditions may develop less immunity to the flu than younger, healthier people. However, experts still recommend vaccination for these high-risk groups as it may still provide some protection against the flu. According to the Centers for Disease Control and Prevention in the United States, one study estimated that flu vaccination lowered the risk of hospitalization in people 50 and older by 61 percent.

You may experience some mild side effects after vaccination. The flu shot may cause soreness, redness, tenderness or swelling at the injection site, as well as a low-grade fever, which should only last for a day or two. The nasal spray vaccine may cause a sore throat, headache or nasal congestion.3, 4

As with any medication, the flu vaccine can cause allergic reactions, including breathing problems, wheezing, hives, hoarseness, weakness, dizziness, paleness or a fast heartbeat. However, allergic reactions are very rare and when they do happen, usually occur within a few minutes to a few hours after the vaccination. If you have a serious reaction to the flu vaccine, call a doctor right away. Tell your doctor about your symptoms, the date and time they started, and when you were vaccinated.1

1. Centers for Disease Control and Prevention. Seasonal Flu Shot. http://www.cdc.gov/flu/about/qa/flushot.htm. Accessed August 20, 2014.
2. Centers for Disease Control and Prevention. Key Facts About Seasonal Flu Vaccine. http://www.cdc.gov/flu/protect/keyfacts.htm. Accessed August 20, 2014.
3. Centers for Disease Control and Prevention. Vaccine Effectiveness – How Well Does the Flu Vaccine Work? http://www.cdc.gov/flu/about/qa/vaccineeffect.htm. Accessed August 19, 2014.
4. Mayo Clinic. Diseases and Conditions: Influenza (flu). http://www.mayoclinic.org/diseases-conditions/flu/in-depth/flu-shots/art-20048000?pg=2. Accessed August 19, 2014.

Information provided is for general background purposes and is not intended as a substitute for medical diagnosis or treatment by a trained professional. You should always consult your physician about any healthcare questions you may have, especially before trying a new medication, diet, fitness program, or approach to healthcare issues.
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