getting a flu shot

Flu Shot: Inactive Vaccine

The vaccine in the flu shot is made from non-infectious, killed viruses, so it is impossible to get the flu from the shot. The flu shot:

  • Is 70% – 90% effective in preventing disease in healthy adults
  • 80% effective in preventing death in the frail, elderly population
  • Can be given to people six months of age and older who do not have an allergic reaction to eggs
  • Takes about two weeks after the shot is given to develop enough antibodies to provide adequate protection from the disease


People nine years of age and older need one shot each season. Children younger than nine years need two shots, given a month apart the first time they receive the vaccine; after that, they only need one shot each season.


Getting influenza disease is much riskier than getting the influenza vaccine; however, a vaccine, like any other medicine, is capable of causing serious problems, such as severe allergic reactions. The risk of the influenza vaccine causing serious harm or death is extremely small.

Mild Reactions

The most frequent side effect of influenza vaccination is soreness at the vaccination site that lasts less than two days. These local reactions typically are mild and rarely interfere with a person’s ability to conduct usual daily activities.

Mild reactions include:

  • Soreness, redness, or swelling where the shot was given
  • Fever (rare)
  • Muscle aches (rare)

Moderate Reactions

Moderate reactions to the flu shot most often affect persons who have had no previous exposure to the influenza virus antigens contained in the vaccine. These reactions begin 6-12 hours after vaccination and can persist for 1-2 days.

Moderate reactions include:

  • Fever
  • Fatigue
  • Muscle pain
  • Headache

Severe Reactions

Life-threatening allergic reactions from influenza vaccine are very rare. If they do occur, it is usually within a few minutes to a few hours after the shot is received. These reactions most likely result from hypersensitivity to residual egg protein.

Persons who have had hives or swelling of the lips or tongue, or who have experienced respiratory distress or collapse after eating eggs, should consult a physician to determine if the vaccine should be administered.

Symptoms of life-threatening allergic reactions include:

  • Hives
  • Swelling of the lips or tongue
  • Swelling around the heart
  • Allergic asthma
  • Difficultly breathing

If these signs occur:

  • Call a doctor, or get the person to a doctor right away.
  • Tell the doctor what happened, the date and time it happened, and when the vaccination was given.
  • Ask the doctor, nurse, or health department to report the reaction by filing a Vaccine Adverse Event Reporting System (VAERS) form. You can file this report through the VAERS website, or by calling 1-800-822-7967. VAERS does not provide medical advice.

Guillain-Barré Syndrome

In 1976, swine flu vaccine was associated with a severe paralytic illness called Guillain-Barré Syndrome (GBS).

Influenza vaccines since then have not been clearly linked to GBS; however, if there is a risk of GBS from current influenza vaccines, it is estimated that it would occur in 1 or 2 cases per million persons vaccinated; much less than the risk of developing severe complications to influenza.

For More Information

  • Call your doctor or nurse. They can give you the vaccine package insert or suggest other sources of information.
  • Contact Boulder County Public Health at 303-413-7500.
  • Contact the Centers for Disease Control and Prevention (CDC) at 1-800-232-2522 (English) or 1-800-232-0233 (Español) or online at

Please Note: All information is general in nature and should not substitute seeking proper medical attention.

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