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The Influenza Vaccine

Influenza Vaccination

Influenza (flu) viruses attack the respiratory system. The usual symptoms include:

  • sore throat
  • headache
  • fever
  • chills
  • muscle aches
  • tiredness

The influenza virus is constantly evolving. That means that the annual flu season is different every year. Some years, most flu cases are mild. Other years, it can be extremely severe. That’s why it’s important to get an annual flu vaccination. Doctors target the vaccine to the viruses that are expected to be most common in the upcoming year.

The flu season can start as early as September and run as late as May.

Everyone over the age of 6 months should be vaccinated annually. However, the people most at risk of serious flu complications are:

  • the very young
  • the elderly
  • pregnant women
  • people with compromised immune systems
  • people with chronic illnesses

These people get priority for vaccination if supplies are low. Serious complications of the flu include pneumonia and dehydration.

Getting Vaccinated

There are two types of flu vaccines available in the United States. The flu shot is an injection of dead (inactivated) virus. It can be given to most people who are 6 months of age or older. The intranasal spray is a live, attenuated vaccine. It contains weakened virus. It should not be given to:

  • children younger than 23 months of age
  • adults over 50
  • pregnant women
  • anyone with a compromised immune system
  • those in close contact with someone who is immunosuppressed
  • people with chronic diseases
  • children or teens on long-term aspirin therapy

Everyone over the age of 6 months should get a flu vaccine once a year. Most people only need one dose. However, some children under 9 may need two doses.

You should be vaccinated as early as possible once the yearly vaccine is made available. It can take several weeks to build immunity.

Who Should Not Get Vaccinated?

Certain people should not get the flu vaccine. This includes anyone with:

  • allergies to chicken eggs or other vaccine components
  • past reactions to the flu vaccine
  • current moderate-to-severe illness
  • a history of Guillain-Barré syndrome (GBS)

While flu vaccination is generally contraindicated in individuals with a history of GBS, in rare instances, the risk of complications from the flu might outweigh the risk of the vaccine. Your doctor may also postpone your flu shot if you’ve had another vaccine within four weeks.

Potential Side Effects

Serious vaccine reactions are extremely rare. However, many people have mild-to-moderate side effects. These include:

  • soreness or swelling at the site of the shot
  • fever
  • sore, red, or itchy eyes
  • runny nose or congestion
  • hoarseness, or cough
  • sore throat
  • headache
  • muscle aches

Serious allergic reactions are extremely rare.

Guillain-Barré Syndrome

In 1976 an inactivated swine flu vaccine was linked to GBS. This is a rare disorder in which the immune system damages the nerves of the body. Since then, the flu vaccine has not been clearly linked to GBS. If the flu vaccine can cause GBS, such cases are extremely rare.  

Content licensed from:

Written by: Amy Boulanger
Medically reviewed on: Nov 17, 2014: Brenda B. Spriggs, MD, MPH, FACP

This feature is for informational purposes only and should not be used to replace the care and information received from your health care provider. Please consult a health care professional with any health concerns you may have.
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