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What is the difference between Flublok and Flublok Quadrivalent?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Feb 25, 2021.

Official answer


Key Points

  • Flublok Quadrivalent, from Sanofi Pasteur, was first approved by the FDA in 2017 and is used to immunize adults 18 years and older against influenza. Flublok was a recombinant trivalent influenza vaccine first licensed by the FDA in 2013.
  • Flublok has now been replaced by Flublok Quadrivalent, which protects against four influenza strains instead of three.
  • Flublok Quadrivalent was the first high antigen content quadrivalent vaccine and contains three times the active ingredients of other quadrivalent vaccines. It is a recombinant vaccine and does not require an egg-grown vaccine virus nor the use of chicken eggs in production.

High antigen content flu vaccines may offer an advantage to people at risk from flu complications, such as seniors or those with lowered immunity. Flublok Quadrivalent has the same purity advantages as trivalent Flublok, containing pure protein made without infectious influenza virus, antibiotics, eggs, thimerosal, preservatives, gelatin or latex.

Common side effects include injection site reactions (tenderness, pain), headache, fatigue, and muscle and bone pain.

Learn more: Flublok vs. Flublok Quadrivalent

What is a quadrivalent flu vaccine?

Quadrivalent vaccines add an additional B strain to the three recommended strains in the trivalent flu vaccine each year, and protect against A and B strains of influenza (4 strains total).

In general, quadrivalent vaccines are now replacing trivalent vaccines in the U.S. market. In the 2020-2021 flu season, only one standard dose trivalent vaccine was available as noted by the CDC (Fluad for those ≥65 yrs).

Even though quadrivalent or high dose vaccines may give you added protection during the flu season, experts say don't delay getting your influenza vaccine if a particular formulation or brand is not available.

Do not use Flublok Quadrivalent is you have ever had a severe allergic reaction (for example, anaphylaxis) to any component of the vaccine.

Related: Why You Should Get Your Flu Vaccine Now: 2020-2021 Updates

This is not all the information you need to know about Flublok Quadrivalent for safe and effective use and does not take the place of talking to your doctor about your treatment. Review the full Flublok Quadrivalent information and side effects here, and discuss this information and any questions you have with your doctor or other health care provider.

  • Recombinant Influenza (Flu) Vaccine. US Centers for Disease Control and Prevention (CDC). Accessed Feb. 25, 2021 at
  • Flublok Quadrivalent (Influenza Vaccine), Sterile Solution for Intramuscular Injection 2020-2021 Formula. [product label]. Revised 7/2020. Accessed Feb 25, 2021 at

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