What Is Influenza?

Influenza (also called the flu) is an upper respiratory infection. It is caused by a strain of the influenza virus. There are many types of influenza viruses but there are two main kinds that infect humans:

• Type A
• Type B

Each year (usually beginning in October), the flu spreads around the world. 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, then putting your hand to your mouth or nose. For most, the flu will cause fever, aches, fatigue, coughing, congestion, loss of appetite, and sore throat. However, some people are more vulnerable to more severe complications which may require hospitalization. Risk factors for severe complications include:

• Age younger than 5 years old or age 65 years and older
• Certain medical conditions, including:
  • Chronic lung condition, such as asthma or COPD
  • Cardiovascular disease
  • Kidney or liver disease
  • Neurological, blood, or metabolic condition, such as diabetes
• 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?

The flu shot is made from an inactivated, killed virus. There are 3 types of flu shots available:

• Regular flu shot (the most common type)—for people aged 6 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. It is the preferred vaccine for healthy children who are 2-8 years of age.

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 6 months and older should get a flu shot. Children 6 months to 8 years of age will need 2 doses of the vaccine. This will help your child build immunity to the virus.

It takes about 2 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.

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
• Have had a severe reaction to the flu vaccine in the past
• Have had Guillain-Barré syndrome (GBS)
• Are currently are 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 2-4 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
  • Have taken influenza antiviral medication within the previous 48 hours
• 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.

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Updated June 17, 2015. Accessed August 10, 2015.

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Influenza vaccine in adults. EBSCO DynaMed website. Available at:
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Influenza vaccine in children. EBSCO DynaMed website. Available at:
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http://www.cdc.gov/flu/protect/keyfacts.htm
Updated October 22, 2014. Accessed August 10, 2015.

People at high risk of developing flu-related complications. Centers for Disease Control and Prevention website. Available at:
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Updated January 8, 2015. Accessed August 10, 2015.

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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(14):1373-1381.

3/5/2012 DynaMed’s Systematic Literature Surveillance
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8/10/2015 DynaMed’s Systematic Literature Surveillance
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