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Influenza Vaccine


Margot L. Savoy

, MD, MPH, Lewis Katz School of Medicine at Temple University

Last full review/revision Jun 2021| Content last modified Jun 2021
Click here for the Professional Version

The influenza virus vaccine helps protect against influenza. Two types of influenza virus, type A and type B, regularly cause seasonal epidemics of influenza in the United States. There are 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. The number of deaths caused by influenza varies greatly from year to year, with about 30,000 to 50,000 deaths annually in the United States (see Centers for Disease Control and Prevention's [CDC] impact of influenza in past years). Occasionally, severe outbreaks, called pandemics, cause even more deaths, especially among young people. In 1918, influenza killed millions of people worldwide.

Administration of Influenza Vaccine

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. It is also available as a nasal spray, which contains live but weakened (attenuated) virus.

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 that virus becomes able to spread from person to person.

An influenza vaccine that has a higher dose of inactivated virus is recommended for people aged 65 and older. It is given as an injection.

The main reason for not giving either influenza vaccine to someone is

  • A serious, life-threatening allergic reaction (such as an anaphylactic reaction) to any component of the vaccine (except for eggs) or to a previous dose of any influenza vaccine

Certain other conditions may affect whether and when people are vaccinated (see also CDC: Who Should NOT Get Vaccinated With These Vaccines?). For example, the nasal spray vaccine, which contains weakened live virus, is not given to the following:

  • Children under age 2 years or people age 50 years and older

  • People with a weakened immune system, such as those with HIV infection

  • Pregnant women

  • People who do not have a functioning spleen

  • Children or adolescents taking aspirin or other drugs that contain salicylates

  • People in close contact with or caring for a person with a severely weakened immune system (unless contact is avoided for 7 days after being given the vaccine)

  • People who have a cochlear implant or a spinal fluid leak

  • People who have taken antiviral medicine for influenza in the last 2 days

  • Children aged 2 to 4 years if they have asthma or have had wheezing or asthma episodes in the past 12 months

If people have a temporary illness, doctors usually wait to give the vaccine until the illness resolves.

The influenza vaccine may contain small amounts of egg because many of the formulations are made from viruses grown in eggs. Recommendations for giving the vaccine to people with an egg allergy vary according to the severity of the allergy:

  • If the only reaction people have had is a rash (hives), a doctor can give them the influenza vaccine.

  • If people have had other reactions to eggs, such as swelling under the skin (angioedema), difficulty breathing, light-headedness, repeated vomiting, and reactions that required an epinephrine shot or other emergency treatment, they can get the influenza vaccine as long as it is given in a health care facility supervised by a health care practitioner who has experience recognizing and managing allergic reactions.

  • If people had a severe allergic reaction after they were given the influenza vaccine, they are not given the influenza vaccine again.

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.

Side Effects of Influenza Vaccine

Occasionally, the injection site becomes sore. Fever and muscle aches occur occasionally.

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, sore throat, and mild wheezing.

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