Influenza Vaccine

There are 3 basic types of influenza virus vaccine:There are 3 basic types of influenza virus vaccine:

• Inactivated influenza vaccine (IIV)

• Live-attenuated influenza vaccine (LAIV)

• Recombinant influenza vaccine (RIV)

Numerals that appear after letter abbreviations indicate valency (the number of influenza virus hemagglutinin antigens represented in the vaccine).

Trivalent vaccines are recommended for the 2024–2025 season (1). The trivalent recombinant influenza vaccine (RIV3) and a cell culture–based vaccine (ccIIV3) do not contain egg protein. The other inactivated and egg-based influenza vaccines available are high-dose vaccine (HD-IIV3) and adjuvant-based vaccine (aIIV3).

Quadrivalent vaccines that covered an additional B virus strain were recommended in previous seasons; however, this strain is no longer deemed to be circulating.

High-dose vaccines are available for patients ≥ 65 years of age. (See also CDC: Different Types of Flu Vaccines.)

Annual vaccination against influenza with an age-appropriate formulation of influenza virus vaccine is recommended for all people ≥ 6 months of age who do not have a contraindication (Annual vaccination against influenza with an age-appropriate formulation of influenza virus vaccine is recommended for all people ≥ 6 months of age who do not have a contraindication (1, 2).

Inactivated influenza vaccine (IIV3) can be administered to all people ≥ 6 months, including pregnant patients.

Recombinant influenza vaccine (RIV3) can be used in people aged 18 to 49 years.

Live-attenuated influenza vaccine (LAIV3) (intranasal influenza vaccine) can be administered to healthy people aged 2 to 49 years who are not pregnant and who do not have immunocompromising conditions. Safety of LAIV3 has not been established in people with disorders that predispose them to complications from influenza, including advanced lung disease or asthma.(intranasal influenza vaccine) can be administered to healthy people aged 2 to 49 years who are not pregnant and who do not have immunocompromising conditions. Safety of LAIV3 has not been established in people with disorders that predispose them to complications from influenza, including advanced lung disease or asthma.

Cell culture-based inactivated influenza vaccine (ccIIV3) is available for people ≥ 6 months.

High-dose inactivated influenza vaccine (HD-IIV3) is available for people ≥ 65 years.

Adjuvanted inactivated influenza vaccine (aIIV3) is also available for people ≥ 65 years.

Adults ≥ 65 years should be given any one of the high-dose vaccines. If none of these is available, then any other age-appropriate influenza vaccine should be used. The high dose is recommended only for those ≥ 65 years.

Health care workers who care for immunocompromised people (ie, those who require care in a protected environment) should be given IIV3 or RIV3 rather than LAIV3 (or they should avoid contact with the immunocompromised people for 7 days after receiving the vaccine).

The main contraindication for any IIV3 is:

• A severe allergic reaction (eg, anaphylaxis) after previous dose of IIV3 or to a vaccine component

Precautions with IIV3 include the following:

• Moderate or severe acute illness with or without fever (vaccination is postponed until illness resolves)

• Guillain-Barré syndrome (GBS) within 6 weeks after a previous dose of an influenza vaccine

• For ccIIV3 only: A history of severe allergic reaction to a previous dose of any other influenza vaccine (ie, any egg-based IIV, RIV, or LAIV)

Contraindications for LAIV3 include the following:

• A severe allergic reaction (eg, anaphylaxis) to any vaccine component or to a previous dose of any influenza vaccine

• Immunocompromise (eg, due to disorders, including HIV infection, or use of immunosuppressants)

• For children and adolescents, concomitant use of aspirin or other salicylates

• Close contact with and/or caring for severely immunocompromised patients who require a protected environment (unless contact is avoided for 7 days after receiving LAIV)

• Pregnancy

• Anatomical or functional asplenia

• Receipt of influenza antiviral medications in the past 48 hours

• Cerebrospinal fluid leak or a cochlear implant

• Age < 2 years or ≥ 50 years

• Age 2 to 4 years if children have asthma or have had wheezing or asthma episodes in the past 12 months

Precautions for LAIV3 include the following:

• Certain chronic disorders such as chronic lung, heart, kidney, liver, hematologic (eg, hemoglobinopathies), or metabolic (eg, diabetes mellitus) disorders

• Asthma in people aged ≥ 5 years

• Moderate or severe acute illness with or without fever (vaccination is postponed until illness resolves)

• Guillain-Barré syndrome within 6 weeks after a previous dose of an influenza vaccine

• Use of specific antiviral medications, eg, amantadine, rimantadine, zanamivir, oseltamivir (these medications are stopped 48 hours before vaccination and are not resumed for 14 days after vaccination)

The main contraindication for RIV3 is:

• A severe allergic reaction (eg, anaphylaxis) after a previous dose of RIV4

Precautions with RIV3 include the following:

• Moderate or severe acute illness with or without fever (vaccination is postponed until illness resolves)

• Guillain-Barré syndrome within 6 weeks after a previous dose of an influenza vaccine

Precautions for patients with suspected egg allergy: Although a history of severe allergic reaction (eg, anaphylaxis) to egg is a labeled contraindication for egg-based IIVs and LAIV, the ACIP recommends that people with a history of egg allergy can receive any licensed influenza vaccine appropriate for their age and health status. Those with reactions beyond urticaria (eg, angioedema, respiratory distress, light-headedness, recurrent emesis) or who required epinephrine should be vaccinated in a medical setting supervised by a health care professional capable of managing severe allergic reactions unless receiving either a cell-culture–based or recombinant vaccine (1).

The influenza vaccine is administered yearly.

For IIV3, the dose is:

• 0.25 mL or 0.50 mL IM (depending on vaccine) for children aged 6 to 35 months

• 0.5 mL IM for people ≥ 3 years

• 0.1 mL intradermally for people aged 18 to 64 years

Children age 6 months to 8 years who have received fewer than 2 influenza vaccine doses or whose influenza vaccination history is unknown should receive 2 doses, separated by at least 4 weeks.

The smaller, intradermal dose can be used to conserve vaccine in times of shortage.

For LAIV3, the dose is 0.1 mL, sprayed into each nostril (total dose is 0.2 mL).

For RIV3, ccIIV3, HD-IIV3, and aIIV3, the dose is 0.5 mL IM.

For IIV3 and RIV3, adverse effects are usually limited to mild pain at the injection site and may include erythema, pain, and swelling at the injection site (see also CDC: Clinical Guidance for Seasonal Influenza Vaccine Safety). Fever, myalgia, and other systemic effects are relatively uncommon; however, people who have been vaccinated may mistakenly think that the vaccine is causing influenza. Such reactions do not contraindicate future vaccination, which should be encouraged.

Multidose vials contain thimerosal, a mercury-based preservative. Public concerns about a possible link between thimerosal and autism have proved unfounded; however single-dose vials, which are thimerosal-free, are available.

For LAIV3, adverse effects are mild; they include rhinorrhea and mild wheezing.

For more information about adverse effects of these vaccines, refer to the prescribing information.

The following English-language resources may be useful. Please note that The Manual is not responsible for the content of these resources.

1. Advisory Committee on Immunization Practices (ACIP): ACIP Recommendations: Influenza (Flu) Vaccine

2. ACIP: Changes in the 2025 Adult Immunization Schedule

3. ACIP: Changes in the 2025 Child and Adolescent Immunization Schedule

4. Centers for Disease Control and Prevention (CDC): Influenza (Flu)

5. European Centre for Disease Prevention and Control (ECDC): Influenza: Recommended vaccinations