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Influenza Vaccine

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The more you know about your health, the better prepared you are to make informed healthcare decisions. Our health library gives you the information you need to take charge of your health.

What Is Influenza?

Influenza (also called the flu) is an upper respiratory infection. It is caused by the influenza virus. Flu strains differ from one year to the next. There are two main kinds that infect humans:

  • Type A
  • Type B

You can get the flu when you breathe in droplets from someone infected with the virus. It can also be spread by touching a contaminated surface and then putting your hand to your mouth or nose.

Each year (usually beginning in October), the flu spreads around the world. Anyone can get it. Some people are at a higher risk of complications. People at higher risk of complications include:

  • Age younger than 5 years old or age 65 years and older
  • Having certain conditions, including:
    • Chronic lung condition, such as asthma
    • Cardiovascular disease
    • Kidney or liver disease
    • Neurological, blood, or metabolic condition, such as diabetes
  • Having a suppressed immune system, such as HIV
  • Being pregnant
  • Being a child or teen who receives long-term aspirin therapy
  • Being American Indian/Alaska Native
  • Being severely obese

Influenza may cause:

  • Fever and chills
  • Severe muscle aches
  • Cough
  • Severe fatigue
  • Headache
  • Decreased appetite, other gastrointestinal symptoms, such as nausea and vomiting
  • Runny nose, nasal congestion
  • Sneezing
  • Sore throat

Treatment may include:

  • Rest
  • Fluids
  • Over-the-counter pain relievers
  • Decongestants
  • Cough suppressants
  • Antiviral medications

What Is the Influenza Vaccine?

The flu shot is made from an inactivated, killed virus. There are three types of flu shots available:

  • Regular flu shot (the most common type)—for people aged six months and older, injected into the muscle (usually in the upper arm)
  • High-dose shot (Fluzone High-Dose)—for people aged 65 years and older, injected into the muscle
  • Intradermal shot (Fluzone Intradermal)—for people aged 18-64 years old, injected into the skin with a smaller needle

There is also a nasal spray (FluMist) made from live, weakened flu viruses. The nasal spray is available for healthy people aged 2-49 years who are not pregnant.

The flu shots and nasal spray contain several influenza viral strains. The type of strains that the vaccine contains change from year to year. The strains are based on which viruses are likely to circulate during that flu season.

Who Should Get Vaccinated and When?

The Centers for Disease Control and Prevention (CDC) recommend that anyone aged six months and older should get a flu shot.

It takes about two weeks for the vaccination to protect you against the flu. Even if you have been vaccinated, you can still get the flu. If you have symptoms, tell your doctor.

You can get the flu anytime during the year. But, flu season typically lasts from October to May. The best time to get vaccinated is as soon as the vaccine is available. This will protect you before the flu comes to your community.

Children younger than 9 years old may need 2 doses of the flu vaccine. This may need to be given to help your child build immunity to the virus. Talk to the doctor to find out how many doses are right for your child.

What Are the Risks Associated With the Influenza Vaccine?

Almost all people who receive the influenza vaccine have no problems. There are certain risks associated with the vaccine. As with any vaccine, there is a small risk of serious problems, including severe allergic reaction.

Side effects associated with the flu shot include:

  • Soreness, redness, and swelling around the injection site
  • Low-grade fever
  • Muscle aches

Side effects associated with the nasal spray vaccine include:

  • Runny nose
  • Headache
  • Vomiting
  • Muscle aches
  • Fever
  • Sore throat
  • Cough
  • Wheezing

Who Should Not Get Vaccinated?

Certain people should talk to their doctor before receiving the influenza vaccine. These include people who:

  • Have any severe (life-threatening) allergies to chicken eggs
    • Note: The vaccine is safe for people with a hives-only allergy to eggs.
  • Have had a severe reaction to the flu vaccine in the past
  • Have had Guillain-Barré syndrome (GBS)
  • Currently are very sick

The following people should not get the nasal spray:

  • Children who:
    • Are aged 24 months or younger
    • Have asthma
    • Are aged 2-4 years who have had wheezing in the past 12 months
    • Have a condition that may increase their risk of flu complications
  • People who:
    • Are aged 50 years and older
    • Have a chronic condition, such as heart disease, lung disease, asthma, kidney or liver disease, metabolic disease, blood disorders
    • Have a nerve or muscle disorder
    • Have a weakened immune system
    • Are in close contact with others who have a weakened immune system
    • Have a nasal condition which makes it difficult to breath
    • Have gotten any other vaccines in the last 4 weeks
  • Pregnant women
  • Children or teens on long-term aspirin therapy

What Other Ways Can Influenza Be Prevented?

Good preventive measures include:

  • Avoid close contact with people who have respiratory infections.
  • Wash your hands often for 15-20 seconds with soap and water. This is especially important to do when you come in contact with someone who is sick. Rubbing alcohol-based cleaners on your hands is also useful.
  • Do not share drinks or personal items.
  • Cover your nose and mouth with a tissue when coughing or sneezing.
  • Do not put your hands near your eyes, mouth, or nose.

What Happens in the Event of an Outbreak?

In the event of an outbreak, the primary focus is to vaccinate as many at risk people as possible, especially those in high priority groups. The use of antiviral medications can reduce the length of the illness when given within two days of onset. Finally, people who are infected should be isolated as much as possible.

Revision Information


  • Flu—United States Department of Health and Human Services

  • Vaccines & Immunizations

    Centers for Disease Control and Prevention

  • Public Health Agency of Canada

  • Vaccines, Blood & Biologics

    United States Food and Drug Administration

  • Advisory Committee on Immunization Practices. Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2013. MMWR Morb Mortal Wkly Rep. 2013;62(18):356.

  • Fluzone high-dose seasonal influenza vaccine. Centers for Disease Control and Prevention website. Available at: Updated July 31, 2014. Accessed August 12, 2014.

  • Influenza in adults. EBSCO DynaMed website. Available at: Updated June 16, 2014. Accessed August 12, 2014.

  • Influenza in children. EBSCO DynaMed website. Available at: Updated June 23, 2014. Accessed August 12, 2014.

  • Influenza vaccine in adults. EBSCO DynaMed website. Available at: Updated June 16, 2014. Accessed August 12, 2014.

  • Influenza vaccine in children. EBSCO DynaMed website. Available at: Updated May 6, 2014. Accessed August 12, 2014.

  • Key facts about seasonal flu vaccine. Centers for Disease Control and Prevention website. Available at: Updated March 6, 2014. Accessed August 12, 2014.

  • People at high risk of developing flu-related complications. Centers for Disease Control and Prevention website. Available at: Updated November 7, 2013. Accessed August 12, 2014.

  • 10/15/2007 DynaMed Systematic Literature Surveillance Nichol KL, Nordin JD, Nelson DB, Mullooly JP, Hak E. Effectiveness of influenza vaccine in the community-dwelling elderly. N Engl J Med. 2007;357:1373-1381.

  • 3/5/2012 DynaMed's Systematic Literature Surveillance FDA approves first quadrivalent vaccine to prevent seasonal influenza. US Food and Drug Administration website. Available at: Published Updated March 15, 2012. Accessed August 12, 2014.

  • 2/11/2014 DynaMed's Systematic Literature Surveillance Bridges C, Coyne-Beasley T. Advistory Committee on Immunization Practices recommended immunization schedules for adults aged 19 years or older: United States, 2014. Ann Intern Med. 2014;160(3):190-197.