Fluzone Quadrivalent is an inactivated flu vaccine made by Sanofi Pasteur that protects against four strains of influenza. It is FDA-approved for anyone 6 months of age and older, making it one of the most widely used flu shots in the United States. The “quadrivalent” part of the name means it targets four virus strains: two influenza A strains (H1N1 and H3N2) and two influenza B strains (one from the Victoria lineage and one from the Yamagata lineage).
How Fluzone Quadrivalent Works
Fluzone Quadrivalent contains inactivated (killed) influenza viruses that have been split apart during manufacturing. Because the viruses are inactivated, they cannot cause the flu. When injected, the viral fragments prompt your immune system to produce antibodies against those four specific flu strains. If you’re later exposed to one of those strains, your immune system recognizes it and responds faster, reducing your chances of getting sick or developing severe illness.
No specific antibody level has been definitively established as a guarantee of protection. In clinical studies, participants who developed a certain threshold of antibodies after vaccination were protected from flu illness roughly 50% of the time. That may sound modest, but even partial protection significantly reduces hospitalizations, complications, and deaths from influenza, especially in vulnerable groups like young children and older adults.
Dosing by Age
The dose you receive depends on your age. Children between 6 and 35 months old get a smaller 0.25 mL injection, which contains 7.5 micrograms of the active protein (called hemagglutinin) per virus strain. Children 3 years and older and all adults receive a full 0.5 mL dose containing 15 micrograms per strain.
Children under 9 years old who are getting a flu vaccine for the first time typically need two doses, spaced about four weeks apart. After that initial series, one dose per season is standard. If a child accidentally receives the smaller pediatric dose instead of the full adult dose, a second pediatric dose can be given on the same visit to make up the difference.
Standard Dose vs. High-Dose
There is also a version called Fluzone High-Dose Quadrivalent, designed specifically for adults 65 and older. Aging weakens the immune response, so the high-dose formulation packs four times more antigen: 60 micrograms of hemagglutinin per strain, totaling 240 micrograms across all four strains, compared to 60 micrograms total in the standard version. This higher concentration produces a stronger antibody response in older adults, which translates to better protection against flu and its complications.
The CDC preferentially recommends higher-dose or adjuvanted flu vaccines for adults 65 and older, though the standard-dose vaccine is still acceptable if those options aren’t available.
Thimerosal and Preservatives
Whether Fluzone Quadrivalent contains thimerosal (a mercury-based preservative) depends on how it’s packaged. Multi-dose vials, which are punctured multiple times to draw out individual doses, contain a small amount of thimerosal to prevent bacterial contamination. Single-dose vials and prefilled syringes do not contain a preservative because they’re used once and discarded.
If avoiding thimerosal matters to you, requesting a prefilled syringe or single-dose vial is a simple solution. Large scientific reviews have consistently found no safety concerns with thimerosal in vaccines, but preservative-free options are widely available.
Egg Allergies and Fluzone
Fluzone Quadrivalent is manufactured using eggs, which means trace amounts of egg protein can be present in the final product. However, current CDC guidelines are clear: people with egg allergies, even those with a history of severe reactions to eggs, can receive any age-appropriate flu vaccine, including egg-based options like Fluzone.
Starting with the 2023-2024 flu season, the CDC dropped its previous recommendation for extra precautions (like extended observation periods) specifically for egg-allergic individuals. The guidance now treats egg allergy the same as any other background: all vaccines should be given in a setting equipped to handle rare allergic reactions, which is standard practice everywhere vaccines are administered.
Common Side Effects
The most common reaction is soreness, redness, or swelling at the injection site. These local effects typically appear within hours and resolve in one to two days. Systemic reactions like low-grade fever, muscle aches, headache, and fatigue can also occur, particularly in younger children and people receiving the high-dose version. These are signs your immune system is responding to the vaccine and are not cause for concern.
Serious allergic reactions to Fluzone Quadrivalent are extremely rare. The vaccine should not be given to anyone who has had a severe allergic reaction to a previous dose of Fluzone or to any of its components.
When and Where to Get It
Fluzone Quadrivalent is reformulated every year to match the influenza strains scientists predict will circulate that season. It typically becomes available in late summer or early fall, and the CDC recommends getting vaccinated by the end of October, though later vaccination still offers benefit throughout flu season. The vaccine is available at pharmacies, doctor’s offices, community health centers, and many workplaces. Protection develops about two weeks after vaccination and generally lasts through the flu season.