Influenza Vaccine
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Influenza Vaccine

En Español (Spanish Version)

What Is Influenza?

Influenza, or "the flu," is an upper respiratory infection caused by the influenza virus. The strains of influenza differ from one year to the next. There are two main kinds of influenza viruses: type A and type B.

People get the flu when they breathe in infected respiratory droplets from someone who is carrying the virus. Influenza can also be spread via contaminated surfaces, such as when people touch a contaminated surface and transfer the virus from their hand to their mouth or nose.

Each winter, influenza spreads around the world. Anyone can get it, but people who live or work in crowded group conditions (eg, nursing home, school, military, day care), newborn babies, the elderly, women in the third trimester of pregnancy, people with diabetes, people with weakened immune systems (eg, cancer patients, AIDS patients, people taking immunosuppressive drugs), and people with chronic conditions (eg, lung, heart, kidney, or blood conditions) are at higher risk of getting the flu.

On average, 5%-20% of the United States population gets the flu each year, more than 200,000 people are hospitalized for flu-related complications, and about 36,000 die from the disease.

Symptoms include:

  • Fever and chills
  • Severe muscle aches
  • Severe fatigue
  • Headache
  • Decreased appetite, other gastrointestinal symptoms (eg, nausea, vomiting)
  • Runny nose, nasal congestion
  • Sneezing
  • Watery eyes, conjunctivitis
  • Sore throat
  • Cough
  • Swollen lymph nodes in the neck

Treatment for the flu may include antiviral medications, bed rest, fluids, over-the-counter pain relievers, decongestants, and cough medicines.

What Is the Influenza Vaccine?

There are two types of influenza vaccines: the "flu shot" and the nasal spray flu vaccine. The flu shot, trivalent influenza vaccine (TIV), is approved for use in people older than six months and is made from an inactivated, or killed, virus. It is given by injection, usually into the arm, and should be refrigerated prior to administration.

The nasal spray flu vaccine, attenuated influenza vaccine (LAIV), is approved for healthy people ages 2-49 years who are not pregnant. It is made from live, weakened flu viruses. It is taken by nasal spray and should be frozen prior to administration.

Both vaccines contain three influenza viruses, based on international surveillance and scientists' estimations of which viruses are likely to circulate in a given year.

In a recent CDC press briefing, it was reported that the vaccine used for the 2007-2008 flu season may not provide adequate protection against the A (H3N2) and B strains. However, vaccination can still weaken the virus and prevent complications.

Who Should Get Vaccinated and When?

Anyone who wants to reduce his or her chances of getting the flu should get vaccinated. However, it is recommended that certain "priority groups" should get vaccinated each year. This becomes important, since there are often shortages of flu vaccines. The priority groups include:

  • Children age six months to five years
  • Children 5 years and over with certain risk factors
  • Pregnant women
  • People age 50 years and older
  • People with certain chronic medical conditions
  • People who live in nursing homes or long-term care facilities
  • Household contacts of any of the above
  • Household contacts and out-of-home caregivers of children younger than six months of age
  • Healthcare workers

The best time to get vaccinated is October or November, but people can—and should—still get vaccinated in December or later. Flu season can begin as early as October and last as late as May.

It is recommended that two doses are administered (separated by 4 weeks or more) to children less than 9 years who are receiving influenza vaccine for the first time or were vaccinated last season for the first time but received only one dose.

What Are the Risks Associated With the Influenza Vaccine?

Almost all people who receive the influenza vaccine have no problems with it. But there are certain risks associated with the influenza vaccine. As with any vaccine, there is a risk of severe problems, including severe allergic reaction.

The risks associated with the flu shot include soreness, redness, or swelling around the injection site; low-grade fever; and aches.

The risks associated with the nasal spray vaccine include runny nose, headache, vomiting, muscle aches, fever, sore throat, and cough.

Who Should Not Get Vaccinated?

Certain people should consult their physician before receiving the influenza vaccine. These include people who have a severe allergy to chicken eggs, people who have had a severe reaction to a prior influenza vaccine, people who developed Guillain-Barre syndrome within six weeks of a previous influenza vaccine, and children younger than six months.

The LAIV vaccine should not be administered to children under five years of age with recurrent wheezing.

People who are moderately or severely ill should wait until they recover to get vaccinated.

What Other Ways Can Influenza Be Prevented Besides Vaccination?

Good preventive measures for influenza include:

  • Practice thorough hand washing, including using alcohol-based gels.
  • Avoid touching eyes or nose prior to washing hands.
  • Avoid biting fingernails.
  • Avoid sharing personal items during the flu season.

What Happens in the Event of an Outbreak?

In the event of an influenza outbreak, vaccinating as many susceptible people as possible, especially those in priority groups, is the primary focus in preventing and controlling further spread of the disease. In addition, the use of antiviral medications (eg, oseltamivir, zanamivir) can reduce the duration of influenza illness when administered within two days of the onset of the illness. Certain antiviral mediations can also be administered before exposure to influenza virus Type A to prevent illness. Finally, people who are infected with influenza should be isolated as much as possible, to prevent spread of the illness.


National Immunization Program
Centers for Disease Control and Prevention

US Food and Drug Administration


Flu. US Centers for Disease Control and Prevention website. Available at: Accessed February 6, 2007.

US Centers for Disease Control and Prevention website. Available at: Accessed February 6, 2007.

*¹1/31/2008 DynaMed's Systematic Literature Surveillance Centers for Disease Control and Prevention. Recommended immunization schedules for persons aged 0-18 years—United States, 2008. MMWR. 2008;57;Q1-Q4. Centers for Disease Control and Prevention, MMWR website. Available at: Updated January 10, 2008. Accessed January 28, 2008.

*²2/13/2008 DynaMed's Systematic Literature Surveillance CDC news conference on influenza. Centers for Disease Control and Prevention website. Available at: Accessed February 13, 2008.

Last reviewed February 2008 by David Horn, MD, FACP

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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