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What is Influenza (Flu) Vaccine?


There are many kinds of influenza vaccines available in the U.S. NVIC encourages consumers to read the vaccine manufacturer's package insert information carefully before receiving influenza vaccine or any vaccine.

Standard-Dose Inactivated Flu Vaccine –

  • The most common flu vaccine is the inactivated (killed) influenza vaccine, which is prepared from the fluids of chick embryos inoculated with a specific type(s) of influenza virus. 1 The strains of flu virus in the vaccine are inactivated with formaldehyde and preserved with thimerosal, which is a mercury derivative.2 (There is a limited supply of thimerosal-free or influenza vaccine with trace amounts of thimerosal and it is supplied in single dose vials, which do not require a preservative). This type of inactivated influenza vaccine is administered by injection into the muscle and contains four (quadrivalent) influenza virus strains - 2 type A and 2 type B strains.3 One quadrivalent flu vaccine, Afluria, is given by jet injector, a medical device which uses high pressure to administer the vaccine.4

High-Dose Quadrivalent Flu Vaccines -

  • The high-dose quadrivalent influenza vaccine, Fluzone, is approved for adults age 65 and older and contains four times the amount of antigen than the standard flu vaccine.5 This vaccine is designed to hyper-stimulate the immune system to produce a stronger antibody response in the elderly.6 High-dose Fluzone is the only high-dose quadrivalent flu vaccine currently available in the U.S.

Recombinant Flu Vaccines -

  • The recombinant flu vaccine, approved for use in 2013, is manufactured through genetic engineering.7 It is produced using insect (armyworm) cells. Flublok is the only recombinant flu vaccine currently licensed in the U.S. and one of two influenza vaccines using alternatives to chicken eggs for production.8 9

Cell-Based Flu Vaccines -

  • Cell-Based flu vaccines differ from standard egg-based flu vaccines because animal cells are used to grow the influenza virus.10 Currently only one cell-based flu vaccine, Flucelvax, is available in the U.S. Licensed in 2012, it is prepared from influenza virus grown in Madin Darby canine kidney cells. This vaccine is approved for use in adults and children age 4 and older.11

Flu Vaccine, Adjuvanted -

  • In 2015, the FDA approved Fluad, the first adjuvanted trivalent flu vaccine containing a squalene oil adjuvant (MF59).12 This vaccine was approved for fast track licensure by the FDA, despite limited data on safety and immunogenicity, with approval based on a single clinical trial of about 1,000 healthy adults over the age of 65. 13 This vaccine became available for the first time to adults over the age of 65 for the 2016-2017 flu season. In 2020, Fluad quadrivalent received FDA approval and will be available for the first time in the fall of 2020. 14

Nasal-Spray Flu Vaccine FluMist -

  • A live-virus nasal flu vaccine, FluMist, was licensed by the FDA in June 2003 and initially approved use in healthy people between the ages of 5 and 49.15 It was subsequently approved by the FDA for use in children as young as two years of age but with precautions.16 In 2014, CDC officials recommended FluMist be the influenza vaccine given to children between 2 and 8 years of age but, in 2016, the CDC recommended that FluMist not be given to children or adults of any age because the vaccine had been found to be ineffective.17 In February 2018, however, the CDC approved a new formulation of FluMist as an option for the 2018-2019 flu season despite a lack of vaccine efficacy studies.18 FluMist is prepared by introducing influenza viruses into eggs where they multiply. FluMist is a live virus vaccine and does not contain any preservatives. The vaccine will be available for the 2020/2021 flu season.19

The majority of influenza vaccines were initially designated as Category B or C pharmaceutical products. This means that adequate and well-controlled studies on pregnant women were not conducted prior to licensure of influenza vaccines and it is not known whether the vaccines can cause fetal harm when administered to a pregnant woman or can affect fertility and the reproduction capacity of a woman. 

In 2015, the FDA removed pregnancy categories and replaced it with a Pregnancy and Lactation Labeling Rule.20 This rule affects all influenza vaccine products submitted after June 30, 2015. As new language is phased in, information on risks associated with vaccinating while pregnant will appear in 8.1 of each vaccine’s product insert under Risk Summary. NVIC encourages pregnant women to read this information carefully prior to receiving influenza vaccine or any other vaccine.

Below are links to the U.S. Food & Drug Administration’s (FDA) website for the most current legally-required licensing information published in manufacturer product package inserts for influenza vaccines available in the U.S.  It is important to understand and read this information carefully prior to receiving a vaccine. Vaccine product package inserts contain important information about ingredients, contraindications, precautions, reported adverse reactions, safety and effectiveness data from pre-licensure clinical trials, use recommendations and more.

Quadrivalent Vaccines - Nasal

Quadrivalent Vaccines – Injected

Trivalent Vaccines - Injected

IMPORTANT NOTE: NVIC encourages you to become fully informed about Influenza and the Influenza vaccine by reading all sections in the Table of Contents, which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child. This information is for educational purposes only and is not intended as medical advice.

« Return to Influenza Table of Contents

« Return to Vaccines & Diseases Table of Contents


1 CDC How Influenza (Flu) Vaccines Are Made Sept. 10, 2020

2 CDC Thimerosal in Flu Vaccine. Oct. 16, 2015

3 CDC Quadrivalent Influenza Vaccine Aug. 21, 2020

4 CDC  Flu Vaccination by Jet Injector Aug. 20, 2020

5 CDC Fluzone High-Dose Seasonal Influenza Vaccine Sept.3, 2020

6 Simonsen L, Reichart T, Viboud C et al., Impact of Influenza Vaccination on Seasonal Mortality in the US Elderly Population Arch Intern Med 2005;165(3):265-272.

7 CDC How Influenza (Flu) Vaccines Are Made - Recombinant Flu Vaccines Sept. 10, 2020

8 CDC Recombinant Influenza (Flu) Vaccine Aug. 21, 2020

9 FDA Flublok Quadrivalent Influenza Vaccine Package Insert Jul. 2, 2020

10 CDC Cell-Based Flu Vaccines. Aug. 21, 2020

11 FDA Flucelvax Package Insert Jul. 2, 2020

12 FDA FLUAD Influenza Vaccine Package Insert Jul. 2, 2020

13 NVIC Questions FDA Fast Tracking of Squalene Adjuvanted Flu Vaccine The Vaccine Reaction Sept. 16, 2016

14 CDC Flu Vaccine With Adjuvant - Flu Vaccine with Adjuvant, brand name FLUAD Aug. 21, 2020

15 FDA June 17, 2003 Approval Letter - Influenza Virus Vaccine Live, Intranasal. June 17, 2003.

16 FDA September 19, 2007 Approval Letter - Influenza Virus Vaccine Live, Intranasal. Sept. 19, 2007.

17 CDC ACIP votes down use of LAIV for 2016-2017 flu season June 22, 2016.

18 Walker, M. ACIP Reinstates FluMist for 2018-2019 Flu Season. MEDPAGE TODAY Feb. 21, 2018

19 FDA FluMist Quadrivalent Influenza Vaccine Package Insert. July 9, 2020

20 FDA Pregnancy and Lactation Labeling (Drugs) Final Rule. Jun. 28, 2018.

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