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Influenza Vaccine
What is it? Overview Usage Side Effects and Warnings

Influenza Vaccine Overview

Written by FoundHealth.

For the latest information on influenza vaccination, visit the Centers for Disease Control and Prevention website at

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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 infecting 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 winter, the flu spreads around the world. Anyone can get it, but some are at a higher risk of complications. Risk factors include:

  • Being 6 months to 4 years old
  • Being 50 years old and older
  • Having certain conditions, including chronic lung condition (eg, asthma ); cardiovascular disease; kidney, liver, neurological, blood, or metabolic condition (eg, diabetes)
  • Having a suppressed immune system (eg, HIV )
  • Being pregnant during the flu season
  • Being 6 months to 18 years old and receiving long-term aspirin therapy (may be at risk for Reye's syndrome )
  • Living in nursing homes or other long-term care facilities
  • Being American Indian/Alaska Native
  • Being severely obese

Symptoms include:

  • Fever and chills
  • Severe muscle aches
  • Cough
  • Severe fatigue
  • Headache
  • Decreased appetite, other gastrointestinal symptoms (eg, nausea, vomiting)
  • Runny nose, nasal congestion
  • Sneezing
  • Watery eyes, conjunctivitis
  • Sore throat
  • Swollen lymph nodes in the neck

Treatment may include:

  • Bed rest
  • Fluids
  • Over-the-counter pain relievers
  • Decongestants
  • Cough suppressants
  • Antiviral medicines

What Is the Influenza Vaccine?

There are two types of influenza vaccines:

  • Flu shot—trivalent influenza vaccine (TIV)
  • Nasal spray (FluMist)—live, attenuated influenza vaccine (LAIV)

The flu shot is approved for use in people older than six months. The shot is made from an inactivated, killed virus. It is given by injection, usually into the arm.

The nasal spray flu vaccine is approved for healthy people aged 2-49 years who are not pregnant. It is made from live, weakened flu viruses. It is sprayed into the nose.

Both vaccines contain three influenza viral strains—type A virus (H3N2), type A virus (H1N1), and a type B virus. The vaccine changes from year to year based on which viruses are likely to circulate in a given flu season.

The strain for the pandemic H1N1 (2009) flu is included in the 2010-11 seasonal flu vaccine. You do not need to have a separate shot for pandemic H1N1 (2009) flu.

If you had either the H1N1 (2009) flu or the shot for it last season, you should still get the flu shot this 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, however, flu season typically begins as early as October and as late as April or May. The best time to get vaccinated is as soon as the vaccine becomes available. Doing so will protect you before the flu comes to your community.

It is recommended that two doses are given (separated by four weeks or more) to children who meet certain criteria:

  • Less than nine years old and receiving influenza vaccine for the first time
  • Vaccinated last season for the first time with flu shot, but with only one dose
  • Children who did not get at least one dose of the 2009 H1N1 flu shot last season

If you are unsure whether your child received any kind of flu vaccine last season, then your child should receive two doses of the flu vaccine.

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.

Adverse effects associated with the flu shot include:

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

Adverse effects associated with the nasal spray vaccine include:

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

Who Should Not Get Vaccinated?

Certain people should consult their doctor before receiving the influenza vaccine. These include:

  • People who have a severe allergy to chicken eggs
  • People who have had a severe reaction to a prior influenza vaccine
  • Children younger than six months
  • People who are sick with a fever

What Other Ways Can Influenza Be Prevented Besides Vaccination?

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, especially 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.
  • Do not bite your nails or put your hands near your eyes, mouth, or nose.

What Happens in the Event of an Outbreak?

In the event of an outbreak, vaccinating as many susceptible people as possible, especially those in priority groups, is the primary focus. In addition, the use of antiviral medications (eg, oseltamivir , zanamivir ) can reduce the duration of the illness when given within two days of onset. Certain antiviral medicines can also be given before exposure to Type A influenza virus to help prevent illness. Finally, people who are infected should be isolated as much as possible.



National Immunization Program
Centers for Disease Control and Prevention

Public Health Agency of Canada

United States Food and Drug Administration


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Key facts about seasonal flu vaccine. Centers for Disease Control and Prevention website. Available at: . Updated December 10, 2008. Accessed September 1, 2009.

Prevention and control of influenza with vaccines. United States Center for Disease Control and Prevention website. Available at: . Accessed September 30, 2010.

Recommended adult immunization schedule—United States, 2010. United States Center for Disease Control and Prevention website. Available at: . Accessed September 30, 2010.

Vaccines and immunizations. Centers for Disease Control and Prevention website. Available at: . Accessed February 6, 2007.

1/31/2008 DynaMed's Systematic Literature Surveillance : Centers for Disease Control and Prevention. Recommended immunization schedules for persons aged 0-18 years—United States, 2008. MMWR. 2008;57;Q1-Q4. Centers for Disease Control and Prevention, MMWR website. Available at: . Updated January 10, 2008. Accessed January 28, 2008.

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8/10/2010 DynaMed's Systematic Literature Surveillance DynaMed's Systematic Literature Surveillance : Fiore A, Uyeki T, Broder K, et al. Prevention and control of influenza with vaccines recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010. MMWR. 2010;59:1-62.


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