mFlusiva becomes the first mRNA influenza vaccine approved by the FDA

1. The FLUENT trial demonstrated 26.6% relative vaccine efficacy over a standard-dose comparator in adults 50 and older, clearing the Food and Drug Administration’s superiority threshold and supporting full approval for adults 50 to 64 with accelerated approval for adults 65 and older contingent on postmarketing confirmatory data.

2. The approval’s regulatory backstory, a rare refusal-to-file notice issued and reversed within two weeks, followed by the departure of both the Center for Biologics Evaluation and Research director and Food and Drug Administration commissioner, is relevant to understanding the unusual postmarketing study commitments attached to the clearance and the uncertainty around Advisory Committee on Immunization Practices convening before the 2026 to 2027 flu season.

The FLUENT phase 3 trial enrolled more than 40,000 adults aged 50 and older and randomized them to receive mFlusiva or a licensed standard-dose inactivated influenza vaccine. The trial was decisive in outcome: mFlusiva demonstrated relative vaccine efficacy 26.6% higher than the standard-dose comparator for the primary endpoint of confirmed symptomatic influenza in adults 50 and older across one flu season. The Food and Drug Administration’s (FDA) superiority threshold was met. The safety profile was consistent with prior influenza vaccine experience, though mFlusiva produced higher rates of injection site pain, fatigue, headache, and myalgia than the comparator, a reactogenicity signature that reflects the more vigorous immune response the messenger ribonucleic acid (mRNA) platform elicits. These are expected and generally short-lived, but patients should be counseled about them before vaccination.

What is clinically interesting about the mFlusiva approval is not only that it happened but that it almost did not. Then-Center for Biologics Evaluation and Research (CBER) director Vinay Prasad issued a refusal-to-file notice in February 2026 arguing Moderna had used an insufficiently rigorous comparator for adults 65 and older, the FDA reversed that position within two weeks after Moderna publicly disclosed the rejection, and both Prasad and then-commissioner Marty Makary subsequently left the agency. The August 5 approval came despite documented opposition from Department of Health and Human Services (HHS) Secretary Robert F. Kennedy Jr. to mRNA vaccine technology and was accompanied by unusually stringent postmarketing study requirements. The regulatory environment around this approval is relevant to clinicians because it shapes the uncertainty around whether the Advisory Committee on Immunization Practices (ACIP) can convene and issue a recommendation before the 2026 to 2027 season, which will determine insurance coverage obligations.

The clinical argument for the mRNA platform in influenza is about the manufacturing calendar. Current flu vaccines require strain selection approximately six months before the season begins. Egg-based and cell-based production processes are slow, and during culture hemagglutinin proteins can acquire mutations that shift the vaccine strain away from the circulating virus. The mRNA process requires only the genetic sequence, compressing the manufacturing timeline to approximately two to three months, improving the probability of a good strain match. The approval also cleared the path for Moderna to resubmit its combination mRNA coronavirus disease 2019 (COVID-19)-influenza vaccine application, which had been withdrawn pending FLUENT efficacy data, a single-injection approach to two major respiratory viruses that would have substantial public health value if the combination data is strong. For patients with egg allergy who have historically navigated egg-free influenza vaccine options, mFlusiva involves no egg exposure in any step of its manufacturing process and is a straightforward first choice.

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