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This is a quadrivalent flu vaccine


For a more detailed explanation of some of the ingredients, see this article:


Active ingredients:

Recombinant proteins from 4 types of influenza (flu) virus:

  • H1N1

  • H3N2

  • Victoria lineage

  • Yamagata lineage

Flublok is a recombinant protein vaccine: Baculovirus (a kind of virus that infects insects but is harmless for humans) is used as a viral vector to insert information for how to make specific proteins into the genome of Spodoptera frugiperda (an insect related to butterflies). These proteins are used as the active ingredient in the vaccine. Because these are proteins, not the whole virus, you cannot get the flu from the vaccine.

This vaccine may contain residual amounts of baculovirus and Spodoptera frugiperda, but these are not harmful to humans.

Inactive ingredients:
Amounts are based on a 0.5 mL dose

Salts: help maintain the stability and pH of the vaccine

Preservatives: prevent bacterial growth and contamination

*Thimerosal is only in multi-dose vials of flu vaccines, which means that one vial of vaccine contains multiple doses. As a result, the container would be reused, so it needs a preservative to prevent contamination between the different uses from the same vial.


Relevant Terms

  • Antibody: a protein made by the immune system in response to a foreign particle (antigen). These proteins circulate in the blood and help defend the body against the substance.  

  • Antigen: This is any substance that triggers the immune system to create antibodies. For example, allergens such as pollen are antigens. 

  • Emulsifier: a substance that helps to keep the ingredients mixed together

  • Genome: all of the genetic information in a cell

  • pH: a measure of how acidic or basic a solution is. The scale ranges from 0 (very acidic) to 14 (very basic). A pH of 7 is neutral.

  • Quadrivalent: protects against four different serotypes of the pathogen (in this case, influenza virus)

  • Viral vector: a version of a virus that cannot replicate or cause disease. It is used to deliver genetic information so that the body can make a protein that acts as an antigen.

Relevant terms


Last updated: Sep. 17, 2023

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