The U.S. Food and Drug Administration has approved Moderna’s mFLUSIVA, making it the first seasonal influenza vaccine in the United States built using messenger RNA technology. The decision is especially notable because the same agency had refused even to begin reviewing Moderna’s application just six months earlier.
Approved on August 5, 2026, mFLUSIVA is intended for adults aged 50 and older. The FDA granted traditional approval for people aged 50 through 64 and accelerated approval for adults 65 and older, with Moderna required to conduct additional studies confirming clinical benefit in the older population.
The reversal turns what had become an unusually public regulatory dispute into a major milestone for Moderna and for mRNA vaccine technology beyond COVID-19.
mFLUSIVA Is the First U.S. mRNA Seasonal Flu Vaccine
Traditional influenza vaccines are generally produced using eggs, cell culture or recombinant protein technology. mFLUSIVA instead uses messenger RNA instructions that direct cells to temporarily produce influenza proteins, prompting the immune system to recognize them.
The technology is conceptually similar to the approach Moderna used for its COVID-19 vaccines, although mFLUSIVA targets influenza rather than SARS-CoV-2.
One potential advantage is production speed. Conventional egg-based vaccine manufacturing takes months, meaning experts must select influenza strains well before the season begins. mRNA vaccines can potentially be redesigned and manufactured more quickly, allowing vaccine composition to respond closer to the emergence of circulating strains.
That does not guarantee a perfect strain match every year. Influenza remains highly unpredictable, and vaccine effectiveness varies depending on which viruses ultimately circulate.
The FDA’s public materials on the vaccine are available through its June 2026 mFLUSIVA advisory committee records.
The Main Trial Included More Than 40,000 Participants
Approval for adults aged 50 through 64 was supported primarily by a large Phase 3 efficacy study involving more than 40,000 adults aged 50 and older.
In that trial, mFLUSIVA produced a 26.6 percent relative improvement in vaccine efficacy against laboratory-confirmed influenza-like illness compared with a licensed standard-dose influenza vaccine. The confidence interval ranged from 16.7 percent to 35.4 percent.
The figure requires careful interpretation. It does not mean mFLUSIVA prevents 26.6 percent of all flu infections or that it is 26.6 percentage points more effective in absolute terms. It means the risk of the study’s influenza endpoint was reduced by approximately 26.6 percent relative to the standard-dose comparator.
The FDA’s analysis reported broadly consistent relative efficacy estimates across different age groups within the trial.
Moderna’s vaccine also produced temporary side effects such as injection-site pain, fatigue, headache, muscle aches and fever somewhat more frequently than conventional flu vaccines. Regulators ultimately concluded that the overall benefit-risk profile supported approval.
Adults 65 and Older Received Accelerated Approval
The regulatory pathway is different for seniors.
For people aged 65 and older, the FDA granted accelerated rather than traditional approval. That allows a product to reach patients based on evidence reasonably expected to predict clinical benefit while additional confirmatory evidence is collected.
The older-adult evidence included a study comparing immune responses generated by mFLUSIVA with those produced by a high-dose influenza vaccine commonly used in seniors. The mRNA vaccine produced strong antibody responses, but regulators still wanted direct confirmation of clinical protection in that population.
Moderna therefore must conduct a postmarketing study.
FDA documents describe a proposed Phase 4 programme comparing mFLUSIVA with a CDC-preferred vaccine in adults aged 65 and older under real-world conditions. One plan discussed by regulators would involve hundreds of thousands of participants across multiple influenza seasons.
Accelerated approval therefore should not be interpreted as regulators simply ignoring missing evidence. It means approval is conditional on Moderna continuing to verify the vaccine’s clinical benefit.
The FDA Had Refused to Review the Vaccine in February
The path to approval was unusually contentious.
On February 10, Moderna announced that the FDA’s Center for Biologics Evaluation and Research had issued a Refusal-to-File letter, meaning the agency would not even begin its formal review of the biologics license application.
The disagreement centered on Moderna’s choice of comparison vaccine.
In the large Phase 3 trial, Moderna compared mRNA-1010, as mFLUSIVA was then known, with a licensed standard-dose influenza vaccine. The FDA argued that this comparator did not represent the “best-available standard of care,” particularly because adults 65 and older are generally offered enhanced influenza vaccines such as high-dose or adjuvanted products.
Importantly, Moderna said the refusal letter did not identify a specific safety or efficacy failure with mRNA-1010 itself.
The company publicly challenged the decision, saying regulators had previously discussed and accepted the trial design before Moderna spent years conducting the study. The dispute attracted considerable attention because refusal-to-file actions at this stage are unusual.
Moderna Changed Its Regulatory Strategy
The impasse did not last long.
Following discussions with the FDA, Moderna revised its application strategy. Rather than seeking the same approval basis across everyone aged 50 and older, the company pursued traditional approval for ages 50 to 64 and accelerated approval for those 65 and above.
The FDA then agreed to begin reviewing the submission and set an August 5 decision date.
That compromise addressed the agency’s concern about the comparator used for older adults without discarding the large efficacy trial entirely.
The episode demonstrates how regulatory disagreements can involve trial design rather than a simple conclusion that a medicine “works” or “does not work.” The question was partly whether Moderna had tested its vaccine against the appropriate benchmark for each age group.
FDA Advisers Ultimately Voted 9–0 in Favor
By June, the regulatory picture had changed dramatically.
The FDA’s Vaccines and Related Biological Products Advisory Committee reviewed mFLUSIVA on June 18. The independent advisers voted 9–0 that the vaccine’s benefits outweighed its risks in adults aged 50 through 64.
They then voted 9–0 again in favor of the benefit-risk profile for adults 65 and older.
The unanimous recommendation did not itself approve the vaccine, but the FDA ultimately followed it.
The agency’s own briefing documents identified the 26.6 percent relative vaccine efficacy result as a key basis for traditional approval in the younger group while acknowledging uncertainties concerning duration of protection and evidence in some special populations.
mRNA Could Help Flu Vaccines Respond Faster to Changing Strains
The broader significance extends beyond one Moderna product.
Seasonal influenza vaccination is fundamentally a race against viral evolution. Global surveillance teams study which strains are circulating and recommend which ones manufacturers should target during the coming season.
For 2026–2027, the FDA’s advisory committee recommended three influenza components covering H1N1, H3N2 and the B/Victoria lineage.
The current U.S. composition can be reviewed through the FDA’s official 2026–2027 influenza vaccine recommendations.
Because mRNA vaccines can potentially move from strain selection to production more rapidly than egg-based products, the platform could eventually allow manufacturers to wait longer before finalizing formulations.
That could reduce the time during which circulating influenza viruses can evolve away from the strains selected for vaccination, although whether that theoretical advantage consistently improves real-world effectiveness will need to be demonstrated over multiple seasons.
Availability May Be Limited at First
FDA approval does not necessarily mean mFLUSIVA will immediately become the dominant flu shot.
Moderna has said some doses could be available through selected retailers, but the company has already missed much of the normal contracting cycle for the 2026–2027 influenza season. Analysts therefore expect the vaccine’s larger commercial impact to emerge during the 2027–2028 season rather than immediately.
Insurance coverage and federal vaccination recommendations may also influence how quickly the product is adopted.
Existing influenza vaccines from manufacturers including Sanofi, GSK and CSL Seqirus will remain available. Adults aged 65 and older already have vaccines specifically recommended for their age group, so mFLUSIVA enters a market with established alternatives rather than replacing them overnight.
The Approval Could Matter for Moderna’s Next Vaccines
For Moderna, mFLUSIVA is strategically important because demand for standalone COVID-19 vaccination has fallen substantially from pandemic-era levels.
The company has been trying to use the same mRNA platform across respiratory diseases, including RSV, influenza and combination vaccines.
Approval of the influenza component could support future efforts to develop a single injection targeting both COVID-19 and flu. Moderna has previously studied such combination products, although each future vaccine would still need appropriate clinical evidence and regulatory review.
The FDA decision also provides the clearest evidence yet that mRNA technology can move beyond COVID-19 into routine seasonal influenza vaccination in the United States.
A Dramatic Regulatory Reversal
The most remarkable part of the mFLUSIVA story may be how quickly its regulatory outlook changed.
In February, the FDA would not open Moderna’s application for review. By June, independent advisers had unanimously concluded that its benefits outweighed its risks. On August 5, the vaccine became the first mRNA seasonal flu shot approved in the United States.
That does not end the scientific questions. Moderna still must confirm clinical benefit in adults 65 and older, real-world effectiveness will need to be measured across changing flu seasons, and conventional vaccines remain important options.
But the approval marks an important expansion of mRNA technology. What became globally familiar during the COVID-19 pandemic is now moving into the annual flu-vaccine market and after an unusually turbulent regulatory journey, Moderna will finally have the opportunity to prove how well it performs outside a clinical trial.
