The influenza virus vaccine helps protect against influenza (flu), which is a viral infection of the lungs and airways. Two types of influenza virus, type A and type B, regularly cause seasonal epidemics of influenza. There are many different strains within each type. The strains of virus that cause influenza outbreaks change each year, so a modified 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. Vaccines that protect against 3 strains are called trivalent, and vaccines that protect against 4 strains are called quadrivalent.
Vaccine Type
There are 3 basic types of influenza virus vaccine:
Inactivated influenza vaccine (IIV): Contains a piece of virus that does not cause infection
Live-attenuated influenza vaccine (LAIV): Contains a weakened (attenuated), live, whole virus that does not cause infection
Recombinant influenza vaccine (RIV): Contains only a specific piece of the virus (such as its protein)
These vaccines contain a harmless version of the virus. The body's immune system is able to recognize and fight the virus if a person becomes infected (see Active immunization).
Influenza vaccines do not cause influenza disease (flu).
Dose of and Recommendations for Influenza Vaccine
The influenza vaccine is given yearly. Influenza epidemics usually begin in late December or midwinter. Therefore, the best time to get the vaccine is in September through November.
IIV and RIV are given as 1 injection into a muscle.
LAIV is a nasal spray and is sprayed once into each nostril.
People who should get this vaccine
The influenza vaccine is recommended for all people 6 months of age and older. (See American Academy of Pediatrics [AAP]: Recommended Child and Adolescent Immunization Schedule for Ages 18 Years or Younger, Centers for Disease Control and Prevention [CDC]: Child and Adolescent Immunization Schedule by Age, and CDC: Recommended Immunizations for Adults Aged 19 Years and Older, United States, 2025.) Which vaccine a person receives depends on their age and other factors, such as whether they have any health problems.
IIV can be given to all people 6 months of age and older, including pregnant people. A high-dose version of IIV is recommended for people 65 years old and older.
RIV can be used in people aged 18 to 49 years old.
LAIV (the nasal spray) can be given to healthy people aged 2 to 49 years old who are not pregnant and who do not have a disorder that weakens their immune system (see below).
People who have an egg allergy may still receive an influenza vaccine. The influenza vaccine may contain small amounts of egg because many of the formulations are made from viruses grown in eggs. According to the CDC, people with a history of egg allergy should receive the influenza vaccine, regardless of the severity of any previous reaction to egg. Any influenza vaccine that is recommended based on the person’s age and health status can be used. People should receive all vaccines, including the influenza vaccine, from health care professionals who have access to the proper equipment needed to recognize and treat a serious allergic reaction.
People who should not get this vaccine
People who have had a serious, life-threatening allergic reaction (such as an anaphylactic reaction) to any component in an influenza vaccine or to a previous dose of any influenza vaccine should not receive a 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 LAIV, which contains weakened live virus, is not given to the following people:
Children under 2 years old or people 50 years old and older
People with a weakened immune system, such as those with HIV infection
Pregnant people
People who do not have a spleen or their spleen does not work well
Children or adolescents taking aspirin or other medications 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 medications for influenza in the last 2 days
Children 2 to 4 years old 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.
Side Effects of Influenza Vaccine
For IIV3 and RIV3, side effects typically include soreness at the injection site. Fever and muscle aches occur occasionally. These side effects may make people think they are developing the flu, but influenza vaccines do not cause influenza.
The LAIV nasal spray sometimes causes a runny nose and mild wheezing.
Whether influenza vaccines increase the risk of developing Guillain-Barré syndrome, a progressive but temporary nerve disorder, is unclear. However, if this rare syndrome develops within 6 weeks after an influenza vaccination, people should talk to their doctor about whether they should receive future vaccinations.
For more information about side effects, see the package inserts.
More Information
The following English-language resources may be useful. Please note that The Manual is not responsible for the content of these resources.
Centers for Disease Control and Prevention (CDC): Live, Attenuated Influenza vaccine information statement
CDC: Inactivated or Recombinant Influenza vaccine information statement
American Academy of Pediatrics (AAP): Recommended Child and Adolescent Immunization Schedule for Ages 18 Years or Younger
European Centre for Disease Prevention and Control (ECDC): Influenza: Recommended vaccinations
