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Influenza Vaccine

By William D. Surkis, MD; Jerome Santoro, MD

The influenza virus vaccine helps protect against influenza. There are two types of influenza virus, type A and type B, and many different strains within each type. The strains of virus that cause influenza outbreaks change each year. Thus, a new vaccine is needed each year. Each year's vaccine is directed against the 3 or 4 strains that scientists predict will be most common in the coming year.

Influenza can be mild, causing fever, aches, and fatigue, but it can be serious. Influenza can cause severe pneumonia, worsening of chronic heart and lung disorders, organ failure, and death. Influenza causes 30,000 to 50,000 deaths annually in the United States. Occasionally, severe outbreaks, called pandemics, cause even more deaths, especially among young people. In 1918, influenza killed millions of people worldwide.


The influenza vaccine is recommended for all people aged 6 months and over.

Influenza vaccine is usually given as an injection of inactivated virus into the muscle, but it is also available as a nasal spray. The nasal spray, which contains live virus, is recommended only for healthy people who are aged 2 to 49 and not pregnant. An influenza vaccine that has a higher dose of inactivated virus and is given as an injection is recommended for people over age 65.

Influenza epidemics usually begin in late December or midwinter. Therefore, the best time to get the vaccine is in September through November. A vaccine against avian influenza ( bird flu) has been developed in case the virus becomes able to spread from person to person.

Did You Know...

  • People with a severe egg allergy may have an allergic reaction to the influenza vaccine because it is made from viruses grown in eggs.

  • Most people with an egg allergy can be given the injected influenza vaccine, but not one that contains live virus.

Side Effects

Occasionally, the injection site becomes sore.

Whether the influenza vaccine increases the risk of developing Guillain-Barré syndrome, a progressive nerve disorder, is unclear. However, if this rare syndrome develops within 6 months after influenza vaccination, people should talk to their doctor about whether future vaccinations are advisable.

The live-virus nasal spray vaccine sometimes causes a runny nose and sore throat.

People who have a severe allergy to eggs may have a severe allergic reaction to the vaccine because many (but not all) of the vaccines are made from viruses grown in eggs.

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