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. It ranges from 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.
For more information, see the Centers for Disease Control and Prevention's (CDC) Live, Intranasal Influenza vaccine information statement and Inactivated Influenza vaccine information statement.
(See also Overview of Immunization.)
The influenza vaccine is recommended for
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 the vaccine or to one of its components
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
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 is present.
If people have a severe allergic reaction after they are given the influenza vaccine, they are not given the influenza vaccine again.
Occasionally, the injection site becomes sore. Fever and muscle aches occur uncommonly.
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.