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Influenza (Flu)
Entry last updated: December 29, 2003
Understanding the Disease

Influenza or the "flu" can affect anyone, and epidemics of flu occur during the winter months nearly every year. The flu is spread through coughing and sneezing, and is highly contagious, especially in childcare centers, schools, and nursing homes.

Flu generally comes on suddenly, and symptoms include muscle aches, fever, chills, headache, cough, and runny nose. Young infants may develop croup. The most frequent complication of flu is pneumonia. Myocarditis (inflammation of the heart) and worsening of chronic lung diseases also may occur.

On average, flu causes approximately 36,000 deaths and 148,000 hospitalizations each year in the United States. More than 90% of flu-related deaths are in people aged 65 years or older.

Available Vaccines

Product: FluMist�
Manufacturer: MedImmune Vaccines��
Year Licensed:�2003

Product: FluShield�
Manufacturer: Wyeth�
Year Licensed: 1961
Note: As of November 2002, Wyeth will stop production of this vaccine; however, supplies of FluShield� may still be in use through the end of the flu season in the spring of 2003.

Product: Fluvirin�
Manufacturer: Evans Vaccines
Year Licensed: 1988

Product: Fluzone�
Manufacturer: Aventis Pasteur
Year Licensed: 1978

For information on the thimerosal content in these vaccines, see the Food and Drug Administration at
or Johns Hopkins University's Institute for Vaccine Safety at

History of the Vaccine

Flu vaccines have been used since 1945. Until recently, all available flu vaccines�were inactivated (killed) virus vaccines. In spite of common belief, inactivated vaccines cannot cause the flu. In June of 2003, a live, attenuated, cold adapted, temperature sensitive, trivalent influenza virus vaccine (FluMist�1) was licensed in the United States. The temperature sensitive type A and B strains of influenza virus contained in FluMist replicate (multiply) in the nasal passages but not in the lower repiratory tract.

Inactivated flu vaccines came in whole-virus and split-virus forms prior to 2001; however, because of fewer side effects, including fever and reactions at the injection site, only split-virus vaccines are currently available in the U.S. Due to the change in the types of virus circulating each year, some of the components of the flu vaccine must be changed as well. The�live, attenuated�vaccine, FluMist, contains no preservatives and is licensed for use in persons ages 5 to 49 years.

1 NNii uses vaccine trade names only for clarity in our presentation of immunization recommendations. NNii does not recommend specific vaccine brands over others.

Who Should and Should Not Receive this Vaccine

Who should receive the inactivated flu vaccine each year?

People (including children over six months of age) at increased risk of developing complications from flu, including those who:

  • Are 50 years old or older (especially those 65 and older)
  • Live in nursing homes or other long-term care facilities
  • Have long-term disorders of the lungs, heart, or circulation (including asthma or cystic fibrosis)
  • Have metabolic diseases (including diabetes)
  • Have kidney disorders
  • Have blood disorders (including anemia or sickle cell disease)
  • Have weakened immune systems (including immunosuppression caused by medications, malignancies, organ transplant,�or HIV infection)
  • Are children who receive long-term aspirin therapy (and therefore have a higher chance of developing Reye syndrome if infected with the flu)
  • Are women who will be in the second or third trimester of pregnancy during the flu season (November to April)
Starting in the 2004/2005 flu season, the Advisory Committee for Immunization Practices at CDC recommends that children ages 6 to 23 months receive an inactivated influenza vaccine.

Who should receive an age-appropriate influenza vaccine each year?

People who can transmit flu viruses to those at high risk for complications, including:

  • Health care providers
  • Employees of nursing homes or assisted living facilities, and providers of home care to people in high-risk groups
  • Emergency response workers
  • Children who are members of households with high-risk persons�

In addition,�an age-appropriate flu�vaccine is encouraged for healthy people six months of age or older, who:

  • Are between the ages of 6 and 23 months (inactivated vaccine)
  • Plan to travel to foreign countries and areas where flu outbreaks may be occurring, such as the Tropics and the Southern Hemisphere from April through September
  • Travel as a part of large organized tourist groups that may include persons from areas of the world where flu viruses are circulating
  • Attend school or college and reside in dormitories, or reside in institutional settings
  • Wish to reduce their risk of becoming ill with flu

Women who are breastfeeding may receive the vaccine. Inactivated influenza vaccine does not affect the safety of mothers who are breastfeeding or the safety of their infants. It is not known if FluMist�1�(MedImmune Vaccines)�is excreted in human milk, therefore, caution is recommended.

Who should not receive�influenza�vaccine:

  • Infants�younger than six months of age
  • Children younger than four years of age should not receive Fluvirin�1 (Evans Vaccines) because it has not been proven effective for them.
  • Children younger than five years of age and adults over 49 years of age should not receive FluMist because safe use in these age groups has not been established.
  • FluMist should not be given to children and adolescents (5-17 years of age) receiving aspirin�or aspirin-containing medications, because of the association with aspirin and wild-type influenza infection.
  • People with a history of asthma or other reactive airway diseases should not be given FluMist because the safety of FluMist in people with a history of these problems has not been established.
  • People with chronic underlying medical conditions that may predispose them to severe flu infections should also not be given FluMist. For these people, the inactivated vaccine is indicated.
  • FluMist should not be given to pregnant women.
  • FluMist should not be given to people with a history of Guillain-Barre syndrome.
  • FluMist should not be given concurrently with other live-virus vaccines.�
  • People who have had an anaphylactic reaction (allergic reactions that cause difficulty breathing, which is often followed by shock) to eggs, egg products,�or other components of the flu vaccine. There are antiviral agents which doctors can prescribe as an alternative for preventing flu in such people.
  • People with acute serious illness with fever
  • People who are moderately or severely ill should consult with their physician before receiving any vaccine
  • Under no circumstances should FluMist be given by injection.

Influenza immunization is recommended by:

  • Advisory Committee on Immunization Practices of the Centers for Disease Control and Prevention
  • American Academy of Pediatrics
  • American Thoracic Society

The summary of adolescent/adult immunization recommendations can be found at:

1 NNii uses vaccine trade names only for clarity in our presentation of immunization recommendations. NNii does not recommend specific vaccine brands over others.

Dose Schedule

When young children (six months until nine years old) are vaccinated against flu for the first time, they should receive two doses of age-appropriate flu vaccine given one month apart. For children 6-35 months of age, �a half dose (0.25 ml) of the inactivated (injected)�influenza vaccine�is�recommended, �in contrast to 0.5 ml which is the usual dose for everyone over�three years of age. In the United States, FluShield� (Wyeth Lederle) and Fluzone� (Aventis Pasteur) may be administered to children as young as six months of age, but Fluvirin� (Evans Vaccines) should only be given to children four years of age and older because its efficacy with younger people has not been demonstrated. FluMist� (MedImmune Vaccines)�should only be given to healthy children and adolescents, ages 5-17 years, and healthly adults, ages 18-49.

When indicated, people nine years old or older should receive one dose of flu vaccine each year. Ideally, people should receive their flu vaccine from the beginning of October through November each year, prior to the flu season, which generally peaks during late December through early March. However, vaccination later in the season is considered worthwhile.

Effectiveness of the Vaccine

Each year, the flu vaccine contains three virus strains, representing the flu viruses likely to circulate in the United States in the upcoming winter.

When the match between the virus strains in the vaccine and the circulating viruses is close, the vaccine prevents illness in up to 90% of healthy adults under the age of 65. Among elderly people, the vaccine is about 30-70% effective in preventing disease, but it is 50-60% effective in preventing hospitalization and 80% effective in preventing flu-related death in the elderly.

The vaccine protects between 45% and 90% of healthy children from getting the flu. Studies have shown that the older and healthier children who have received the flu vaccine are, the more likely they will be protected. Flu vaccination has also been shown to decrease middle ear infections among young children by about 30%.

Known Side Effects

The majority of those immunized against flu will have no adverse reactions. Of those who do have a side effect, most will have soreness or tenderness at the injection site. Fewer than 1% of�adults immunized will also experience fever, chills, or a general sense of feeling unwell that lasts one to two days.� Children are more likely to experience these symptoms.

In very rare cases (far less than 1 out of 10,000), serious reactions can occur. People who have an allergy to eggs (which are used in making the vaccine) or any component of the vaccine are at greater risk for a serious allergic reaction. If you or your child has developed hives or swollen lips or tongue; has had trouble breathing; or has collapsed after eating eggs or receiving a previous dose of influenza vaccine, consult with your healthcare provider to see if the vaccine should be given.�

Because of a slight increase in the frequency of Guillain-Barre Syndrome [GBS]�(a progressive disorder affecting the nervous system) associated with the 1976 swine flu vaccine, subsequent flu vaccines have been closely monitored. Other flu vaccines have not been shown to cause GBS. Even so, if a person is not at high�risk for getting complications from flu, a person who developed GBS within six weeks of a previous flu shot should avoid subsequent flu shots. If the risk from flu is high, they should be vaccinated with an age-appropriate inactivated influenza vaccine because the�established�benefits of the vaccine�justify vaccination.�FluMist, the live, attenuated, nasally administered vaccine, should not be given to individuals who have a history of GBS because safety in those persons has not been investigated.

Related Issues

Flu vaccines given as a nasal spray are being used for adults in Russia, and have been under development in the U.S. since the 1960's. FluMist�,�a live-attenuated influenza virus�vaccine (LAIV) administered as a nasal spray was approved by the FDA in June of 2003. It is the first nasally administered vaccine to be marketed in the United States and the first live virus influenza vaccine approved in the U.S. The possible�advantages of this type of vaccine are that it is easy to administer, has the potential to induce a broad mucosal and systemic�immune response, and�has been shown to be approximately 87% effective in children. However, no study has directly compared the efficacy or effectiveness of trivalent inactivated vaccines and trivalent LAIV.

When there is a major change in the flu virus strain from one year to the next, epidemics or pandemics (world outbreaks) can occur. During the 20th century, there were four major pandemics; the worst caused 21 million deaths worldwide and 500,000 deaths in the U.S. from 1918 to 1919. Between 1957 and 1986 there were 19 different flu epidemics in the U.S.; several of the most recent caused more than 40,000 deaths.

Key References and Sources of Additional Iniformation
  • American Academy of Pediatrics, Committee on Infectious Diseases. (2000). Influenza. In Red Book: Report of the Committee on Infectious Diseases (25th ed., pp. 351-359). Elk Grove Village, IL: Author.
  • Centers for Disease Control and Prevention (CDC). (2002). Influenza vaccine: What you need to know: 2002-2003 [Vaccine Information Statement (VIS)]. Available on-line:
  • CDC. (2002). Prevention and control of influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). Morbidity and Mortality Weekly Report, 51 (RR-03), 1-31. Available online:
  • CDC, National Immunization Program. (2002). Influenza. In Epidemiology and prevention of vaccine-preventable diseases ("The Pink Book") (7th ed., pp. 190-204). Atlanta: Author. Available online:
  • Cohen GM and Nettleman MD. (2000). Economic impact of influenza vaccination in preschool children. Pediatrics, 106(5), 973-976.
  • Izurieta HS, Thompson WW, Kramarz P, Shay DK, Davis RL, DeStefano F, Black S, Shinefield H, and Fukuda K. (2000). Influenza and the rates of hospitalization for respiratory disease among infants and young children. New England Journal of Medicine, 342(4), 232-239.
  • Kilbourne ED and Arden NH. (1999). Inactivated influenza vaccines. In Plotkin and Orenstein (Eds.). Vaccines (3rd ed., pp. 531-552). Philadelphia: W.B. Saunders Company.
  • Morens DM. (2003). Influenza-related mortality: Considerations for practice and public health. Journal of the American Medical Association, 289(2), 227.
  • National Institute of Allergy and Infectious Diseases, Division of Microbiology and Infectious Diseases. (2002). The Jordan Report 20th Anniversary: Accelerated development of vaccines 2002. Bethesda, MD: Author. Available online:
  • Neuzil KM, Mellen BG, Wright PF, Mitchel EF, and Griffin MR. (2000). The effect of influenza on hospitalizations, outpatient visits, and courses of antibiotics in children. New England Journal of Medicine, 342(4), 225-231.

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© Copyright National Network for Immunization Information. The information contained in the National Network for Immunization Information Web site should not be used as a substitute for the medical care and advice of your health care provider. There may be variations in treatment that your health care provider may recommend based on individual facts and circumstances.