Influenza immunization (vaccine)

Alternative names
Vaccine - influenza; Flu shot immunization; Flu vaccine

This vaccination protects people from contracting influenza, a viral illness affecting the respiratory tract.


In the U.S., “flu” outbreaks typically occur in winter months. Symptoms of influenza include fever, chills, muscle aches, and cough. Although the illness usually only lasts 3 to 7 days, some people have more severe cases or complications that require hospitalization.

Thousands of people in the U.S. die each year as the result of the flu or its complications. Most of those who die are the elderly, young children, or people with compromised immune systems.

The viruses that cause influenza change rapidly. Influenza vaccines are developed each year to protect people from the strains expected to be most prevalent. All the viruses in the vaccine are killed, so it is not possible to get the flu from the vaccine. However, some people do experience a low-grade fever afterwards as their immune systems gear up to recognize the virus.

Flu vaccination is generally given at the beginning of the “flu season” - usually late October or early November in the United States. People traveling to other countries should be aware that influenza may occur at different times of the year in other areas.


The vaccine is recommended for high-risk people 6 months and older as well as those in contact with them (including household contacts):

  • all people 50 years or older  
  • adults and children with chronic lung or heart disease  
  • people with sickle cell anemia and other hemoglobinopathies  
  • residents of nursing homes (extended care facilities)  
  • residents of any institution housing people with chronic health problems  
  • people with kidney disease, anemia, severe asthma, or chronic metabolic illnesses (such as diabetes or chronic liver disease)  
  • people with immunological deficits (including those with cancer or HIV/AIDS)  
  • people receiving long-term treatment with steroids for any condition  
  • pregnant women who will be past the 3rd month of pregnancy during the flu season (you may want to consider requesting the mercury-free flu vaccine)  
  • children and teenagers receiving long-term aspirin therapy

The vaccine is also recommended for health care providers attending to high-risk people:

  • health care personnel in hospital, outpatient, extended care facilities, and home health care  
  • family members attending to the needs of high-risk people in their home

The influenza vaccine is encouraged for:

  • healthy children 6-23 months of age and their contacts/caretakers  
  • household contacts/caretakers of infants less than 6 months of age  
  • people who provide essential community services  
  • people living in dormitories or other crowded conditions  
  • anyone who wants to reduce their change of getting influenza

Children under age 9 require two shots one month apart the first time that they receive influenza vaccine. Other people require a single shot each year.


Most people achieve protection from influenza vaccine approximately 2 weeks after receiving the immunization.

Immunization of high-risk people decreases many potential deaths from influenza. Immunization of those caring for high-risk people decreases the potential of spreading the “flu” from otherwise healthy people to those who are at higher risk of complications.


Most people have no side effects from the influenza vaccine. Soreness at the injection site or minor aches and low grade fever may be present for several days.

Unlike the swine flu vaccine used in1976, flu vaccines in recent years have shown no association with Guillain-Barre syndrome (GBS) in children, and an extremely small increase in the risk of GBS in adults (approximately 1 more case per million persons immunized). This risk is far outweighed by the amount of severe influenza prevented by immunization.

As is the case with any drug or vaccine, there is a remote possibility that allergic reactions, more serious reactions, or even death may occur after receiving the flu shot.


Influenza vaccine should be withheld or only given to the following after consultation with the primary care provider:

  • persons with a severe allergic reaction to chickens or egg protein  
  • people with a fever or illness that is more than “just a cold”  
  • anyone who has exhibited a moderate to severe reaction after a previous influenza shot  
  • women who are, or might be, in the first trimester of pregnancy (first 3 months of pregnancy)  
  • anyone who has ever been paralyzed due to Guillain-Barre Syndrome


Watch for and be familiar with how to treat minor side effects, such as injection site tenderness or low grade fever.


  • uncertain if the influenza vaccine should be delayed, withheld, or given to a specific person  
  • moderate or serious adverse effects appear after the influenza injection has been given  
  • there are any questions or concerns related to the influenza immunization


Johns Hopkins patient information

Last revised: December 8, 2012
by Armen E. Martirosyan, M.D.

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.