The FDA Said No to Moderna's mRNA Flu Shot in February. By August, the Two Men Who Said No Were Gone.

The FDA does not often refuse to read an application. Declining to file a completed submission is the regulatory equivalent of handing the paperwork back at the door, and the agency reserves it for cases where something fundamental is missing. In February 2026 it did exactly that to Moderna’s mRNA flu shot, mFLUSIVA, and the reason was specific: the company had tested the vaccine against an ordinary standard-dose flu shot, not against the stronger formulations that Americans over 65 are actually steered toward. Barely a week later, after a closed-door meeting and a wave of public pressure, the agency reversed the filing decision. By the time mFLUSIVA won full approval on August 5, the two officials who had presided over that refusal were gone from the building, and the shot had been cleared for the very age group where its evidence was thinnest.

To feel the whiplash, back up one year. On August 5, 2025, exactly twelve months before the approval, HHS Secretary Robert F. Kennedy Jr. canceled nearly $500 million across 22 mRNA vaccine projects run through BARDA, a Moderna bird-flu award among them. The reason he gave was about performance: “the data show these vaccines fail to protect effectively against upper respiratory infections like COVID and flu.” Agree with him or not, that was a claim aimed squarely at the mRNA platform’s ability to stop a respiratory virus, the exact job a flu shot is built to do. One year later, the department he runs licensed the first mRNA flu vaccine in American history.

The politics of that reversal will get the attention. The evidence underneath it deserves more, because the evidence barely moved.

The comparator was the shot seniors are steered away from

Older immune systems answer a flu shot weakly. The decline has a name, immunosenescence, and it is why the CDC since 2022 has preferentially recommended that adults 65 and older get one of three enhanced formulations, high-dose, adjuvanted, or recombinant, rather than a standard-dose shot. Those vaccines are built to punch through the fading response, and any new shot that wants a place in that population has to clear the bar they already set.

Moderna’s pivotal trial did not test against that bar. Enrolling more than 40,000 adults aged 50 and older, it measured mFLUSIVA against a licensed standard-dose comparator and reported a relative reduction of 26.6 percent in flu-like illness. The figure is real, and it is also 26.6 percent better than the very shot the enhanced-vaccine guidance exists to move seniors off of. Look at the absolute rates and the gap shrinks further: flu turned up in 2.0 percent of the people who got mFLUSIVA and 2.8 percent of those who got the standard shot, a difference of 0.8 percentage points. Against the high-dose, adjuvanted, or recombinant vaccines that older Americans are actually pointed toward, the trial offers nothing, because that comparison was never run.

The FDA’s own scientists caught this immediately. The refusal-to-file notice that CBER issued in February, under then-director Vinay Prasad, said as much in regulatory language: the application lacked an adequate, well-controlled study because its comparator did not reflect the best-available standard of care. Commissioner Marty Makary’s FDA backed that harder line. It was not obstruction. It was the correct scientific question, asked by the office whose job is to ask it: does this shot beat what the most vulnerable patients already get?

What changed was not the data

The refusal held for barely a week. After a formal meeting with Moderna and what the agency called a revised regulatory approach, the FDA agreed to file the application after all. Moderna did not run the missing head-to-head trial in seniors. It restructured the request instead: full approval for the younger 50-to-64 band, and accelerated approval for the 65-and-older group, contingent on a postmarketing study to be run over the next two flu seasons. The agency agreed to license the vaccine for the population where the evidence was thinnest, on the condition that the proof arrive after the shots did.

Then the officials who had raised the objection left. Prasad departed at the end of April, and Makary followed in the run of resignations that came after him. By the trade press’s own account, their interim replacements took a friendlier posture toward several previously rejected applications, mFLUSIVA among them. In June, a reconstituted advisory committee voted 9 to 0 that the shot’s benefits outweighed its risks for adults 50 and older, a vote taken on a dossier that still held no comparison against the senior standard of care. The application that could not clear the bar in February cleared it in August. What moved in between was not the evidence. It was the people reading it.

The safety signal that was never pinned down

None of this arrives clean. mFLUSIVA will be judged against the safety record of the mRNA COVID shots that built the platform, and that record has loose ends the approval does not tie off. Start with the trial itself: mFLUSIVA produced more side effects than the comparator, mostly fatigue, joint pain, and muscle aches, the reactogenicity penalty the platform has carried from the start. The harder question is myocarditis, and even in the published data the signal is wide and unsettled. A Lancet meta-analysis of mRNA COVID vaccines in children put the myocarditis risk ratio at 4.6, with a confidence interval running from 0.1 to 156.1, an interval so vast it reaches from protective to catastrophic and settles nothing. That estimate is for a different product in a younger group, and it is low-certainty on its own terms, which is exactly the point: the platform’s rarest serious risk has never been bounded with confidence, and mFLUSIVA inherits that uncertainty rather than resolving it.

There is also the matter of who paid. The pivotal COVID trial that launched this platform, BNT162b2 through six months, was funded by its own manufacturers, the same structure that produced mFLUSIVA’s efficacy number. When the company that sells the product also runs the study that clears it, the case for independent confirmation gets stronger, not weaker, and independent confirmation is precisely what the deferred postmarketing study is supposed to supply.

So watch that study. Moderna now has two flu seasons to show that mFLUSIVA actually beats the high-dose, adjuvanted, and recombinant shots that Americans over 65 already receive, the exact comparison Prasad’s CBER demanded before the demand became inconvenient. If the data come back and the shot wins, the approval will look like foresight and the reform-era caution will look like delay. If they do not, the FDA will have licensed a product for its most vulnerable users on the strength of a comparison built to flatter it, and the people who caught that in February will already be gone. The results are due after two seasons in the field. The shots are going into arms now.

Sources

1. STAT – FDA approves Moderna’s mFLUSIVA, the first mRNA flu vaccine (Aug 5, 2026)
2. CBS News – FDA approves Moderna’s mRNA flu vaccine after initial pushback
3. BioPharma Dive – FDA approves Moderna’s mRNA flu vaccine
4. NEJM – Efficacy and Safety of an mRNA Seasonal Influenza Vaccine in Adults (Moderna mRNA-1010, manufacturer-funded, 2026)
5. CDC – Director adopts preference for specific flu vaccines for seniors (2022)
6. BioSpace – RFK Jr. axes 22 mRNA vaccine projects under BARDA (Aug 5, 2025)
7. The Lancet Child & Adolescent Health – Piechotta et al., safety and effectiveness of COVID-19 vaccines in children 5–11 (2023)
8. NEJM – Thomas et al., safety and efficacy of BNT162b2 through 6 months (manufacturer-funded, 2021)