Flublok Quadrivalent is a recombinant flu vaccine approved for adults 18 and older. Unlike traditional flu shots, it’s made without eggs, without growing live flu virus, and without the mutations that can weaken conventional vaccines during manufacturing. It contains three times the active protein of a standard-dose flu shot, with 135 micrograms total compared to the typical 45 micrograms.
How Flublok Differs From Traditional Flu Shots
Most flu vaccines start by growing influenza virus inside chicken eggs or animal cells, then inactivating or weakening that virus for injection. Flublok skips this entirely. Instead, it uses recombinant DNA technology: scientists isolate the genetic instructions for the key protein on the flu virus’s surface (called hemagglutinin, the “H” in names like H1N1) and insert those instructions into insect cells grown in a lab. The insect cells produce large quantities of that protein, which is then purified and packaged into the vaccine.
This matters for two reasons. First, when flu viruses are grown in eggs, they sometimes mutate to better survive in that environment. Those mutations can change the virus’s surface protein just enough to reduce how well the finished vaccine matches the circulating strain. Flublok avoids this problem because no live virus is involved at any stage. Second, the process isn’t dependent on a supply of millions of fertilized chicken eggs, which means production can ramp up faster during a pandemic or unexpected shortage.
What “Quadrivalent” and “Trivalent” Mean Now
Flublok was originally designed to protect against four flu strains (quadrivalent), covering two influenza A subtypes and two influenza B lineages. However, one of the two B lineages, called B/Yamagata, has not been detected in circulation since early 2020. As a result, the World Health Organization recommended removing it from vaccines. For the 2025-2026 season, Flublok contains three strains despite still carrying the “Quadrivalent” trade name: an H1N1 strain, an H3N2 strain, and a single B/Victoria lineage strain. Each strain contributes 45 micrograms of hemagglutinin protein, totaling 135 micrograms per dose.
Higher Protein Content Than Standard Vaccines
A standard-dose inactivated flu vaccine contains 15 micrograms of hemagglutinin per strain. Flublok delivers 45 micrograms per strain, three times that amount. This higher antigen load is built into the vaccine’s design, not an “extra strength” add-on. In a large clinical trial during the 2014-2015 flu season involving nearly 9,000 adults aged 50 and older, Flublok showed 30% greater relative effectiveness against lab-confirmed flu illness compared to a standard inactivated flu vaccine. That trial, reviewed by the FDA, randomized participants equally between the two vaccines and confirmed the cases with laboratory testing.
This higher protein content makes Flublok particularly relevant for older adults, whose immune systems typically mount a weaker response to standard-dose vaccines. It’s one of several enhanced flu vaccine options available to people 65 and older, alongside high-dose and adjuvanted formulations.
Who Can Get Flublok
The FDA has approved Flublok Quadrivalent for anyone 18 years of age and older. It is not approved for children or adolescents. It’s given as a single 0.5 mL intramuscular injection, typically in the upper arm, once per flu season.
Because the manufacturing process uses no eggs at any stage, Flublok is a straightforward option for people with egg allergies. Traditional flu vaccines may contain trace amounts of egg protein, which, while generally considered safe even for most egg-allergic individuals, can be a source of concern. Flublok eliminates that variable entirely.
Side Effects
The side effect profile is similar to other injectable flu vaccines. The most commonly reported reactions in clinical trials were pain, tenderness, or redness at the injection site, along with headache, fatigue, and muscle aches. These are typical immune responses and generally resolve within one to two days. Serious adverse events were rare in clinical studies.
Because Flublok delivers a higher dose of protein than standard vaccines, some people experience slightly more noticeable soreness at the injection site. This is comparable to what’s reported with other enhanced flu vaccines.
How It’s Stored and Administered
Flublok requires standard vaccine refrigeration between 2° and 8° C (36° to 46° F). It should never be frozen, and any dose that has been frozen must be discarded. It comes as a prefilled syringe, and your provider administers it as a single intramuscular injection. No reconstitution or mixing is required.
The 2025-2026 Strain Composition
For the current season, Flublok and other cell- or recombinant-based vaccines use slightly different reference strains than egg-based vaccines, because they don’t need to account for egg adaptation. The 2025-2026 Flublok formulation targets an A/Wisconsin/67/2022-like virus (H1N1), an A/District of Columbia/27/2023-like virus (H3N2), and a B/Austria/1359417/2021-like virus from the B/Victoria lineage. Egg-based vaccines target the same general categories of flu but use different reference strains that have been adapted for egg growth.
This distinction is part of why recombinant vaccines like Flublok may offer a closer match to the viruses actually circulating in the community. The protein in the vaccine is identical to what was selected by health authorities, with no egg-adapted mutations altering it along the way.