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by McCoy K
(Flu Vaccine)

What Is Influenza?

Influenza is an upper respiratory infection known as the flu. It is caused by a strain of the influenza virus. There are many types of these viruses but the 2 that most often infect humans include:

  • Type A
  • Type B

Each year the flu spreads around the world. It spreads most often from person to person. Someone who is infected releases the virus through droplets in sneezes or coughs. Others inhale these droplets, then become sick. The virus can also land on a surface. Someone can become infected if they touch the surface then touch their mouth or nose. For most, the flu will cause fever, aches, fatigue, coughing, stuffiness, and sore throat. Some people have a higher risk of a severe infection. It may lead to hospital care. Risk factors for severe complications include:

  • Age younger than 5 years or age 65 years and older
  • Certain medical conditions, including:
  • Suppressed immune system, such as those with HIV, cancer, or chronic steroid use
  • Current pregnancy
  • Long-term aspirin therapy in people under 19 years old
  • American Indian and Alaskan Native ancestry
  • Severe obesity

What Is the Influenza Vaccine (Flu Shot)?

There are different types of flu vaccine. There are 3 types of flu shots available:

  • Inactivated influenza vaccine—dead, inactive virus. Can be given through shots.
  • Recombinant influenza vaccine—a substance that mimics part of the virus. Can be used for people with egg allergy. Can be given through shots.
  • Live attenuated influenza vaccine—weakened part of the virus. Given only through nose spray.

Vaccines let the body prepare for the flu virus. Within a few days of the vaccine, the body makes tools to help find and fight any flu virus that enters the body. A doctor will help to find which shots will be best for each person. Age and overall health will play a role in the decision. The shots need to be given each year. The strains included in the vaccine will change from year to year. The strains are based on which viruses are likely to spread during that flu season. You may still get the flu if you get a flu strain that was not included in the vaccine. However, you may have a less severe infection. If you have symptoms, tell your doctor.

Who Should Get Vaccinated and When?

The Centers for Disease Control and Prevention (CDC) recommend that anyone aged 6 months and older should get a flu shot. It should be done every year.

It takes about 2 weeks for the vaccine to have full effect. The best time to get a flu shot is as soon as the shot is available. This will protect you before the flu comes to your community.

What Are the Risks Associated With the Influenza Vaccine?

The flu shot is safe for almost all people. There is a small risk of serious problems such as severe allergic reaction.

Minor side effects associated with the flu shot include:

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

Minor 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?

There is a higher risk of problems from the flu shot in people with 1 or more of the following:

  • Have any severe (life-threatening) allergies to chicken eggs
  • Have had a severe reaction to the flu vaccine in the past
  • Have had Guillain-Barré syndrome (GBS)
  • Are currently very sick with a fever

The following people should not get the nasal spray:

  • Children who:
    • Are aged 24 months or younger
    • Have asthma
    • Are aged 2 to 4 years who have had wheezing in the past 12 months
    • Have a condition that may increase their risk of flu complications
  • Adults aged 50 years and older
  • Adults with:
    • Chronic condition, such as heart disease, lung disease, asthma, kidney or liver disease, metabolic disease, blood disorders
    • Nerve or muscle disorder
    • Weakened immune system
    • Close contact with others who have a weakened immune system
    • Nose issues which makes it difficult to breath
    • Other vaccines in the last 4 weeks
    • Influenza antiviral medicine within the previous 48 hours
  • Pregnant women
  • Children or teens on long-term aspirin therapy

What Other Ways Can Influenza Be Prevented?

Habits that may decrease your exposure to the flu virus include:

  • Avoid close contact with people who have cold or flu.
  • Wash your hands often. This is most important to do after contact with someone who is sick. Alcohol-based cleaners on your hands is also useful.
  • Do not share drinks or personal items.
  • Do not put your hands near your eyes, mouth, or nose.

To reduce spread of flu to others, cover your nose and mouth when you cough or sneeze. Use your elbow or a tissue as cover.

What Happens in the Event of an Outbreak?

If an outbreak has started in your community, get a flu shot if you have not had one. The more people who have had flu shots the safer the community will be. Viruses will have a harder time passing from person to person if most have had the flu shot. Follow other precautions like washing your hands even if you have had the flu shot.


Centers for Disease Control and Prevention  https://www.cdc.gov 

Public Health Agency of Canada  https://www. canada.ca 

US Food & Drug Administration  https://www.fda.gov 


Key factors about influenza vaccine. Centers for Disease Control and Prevention website. Available at: https://www.cdc.gov/flu/prevent/keyfacts.htm#match. Updated September 16, 2018. Accessed May 10, 2019.

Influenza in adults. EBSCO DynaMed website. Available at:  https://www.dynamed.com/condition/influenza-in-adults . Updated April 18, 2019. Accessed May 10, 2019.

Influenza in children. EBSCO DynaMed website. Available at:  https://www.dynamed.com/condition/influenza-in-children . Updated October 29, 2018. Accessed May 10, 2019.

Seasonal influenza vaccines in adults. EBSCO DynaMed website. Available at:  https://www.dynamed.com/prevention/seasonal-influenza-vaccines-in-adults . Updated September 13, 2018. Accessed May 10, 2019.

Seasonal influenza vaccines in children. EBSCO DynaMed website. Available at:  https://www.dynamed.com/prevention/seasonal-influenza-vaccines-in-children . Updated September 26, 2018. Accessed May 10, 2019.

Seasonal influenza vaccines in the elderly. EBSCO DynaMed website. Available at:  https://www.dynamed.com/prevention/seasonal-influenza-vaccine-in-older-adults . Updated November 28, 2018. Accessed May 10, 2019.

Revision Information

  • Reviewer: EBSCO Medical Review Board Marcie L. Sidman, MD
  • Review Date: 09/2018
  • Update Date: 08/12/2020