The FDA Can't Decide How It Feels About Moderna's Flu Shot
The FDA refused Moderna's mRNA flu vaccine application, reversed itself two weeks later, and now its briefing documents are sending both positive and cautious signals ahead of a critical advisory panel vote. The outcome could determine whether mRNA technology breaks into the $50B+ flu vaccine market.
Imagine asking your professor if your thesis topic is okay, getting a thumbs up, writing the whole paper, and then being told at submission that your methodology doesn't count. Now imagine them changing their mind again two weeks later.
That's roughly what happened between Moderna and the FDA over the past year. And heading into a critical advisory committee meeting on June 18, the mixed signals are still coming.
A Regulatory Soap Opera
Let's rewind. Moderna has been developing mRNA-1010, a seasonal flu vaccine built on the same mRNA platform that powered its blockbuster COVID shot. The company ran a large Phase 3 trial comparing mRNA-1010 against a standard-dose flu vaccine in adults 50 and older. Before the trial even started, the FDA's Center for Biologics (CBER) reviewed the study design and said the standard-dose comparator was acceptable.
The trial ran. The data came back looking good. Moderna filed its application in November 2025.
Then, on February 3, 2026, the FDA issued a refusal-to-file letter. The reason? The comparator vaccine "does not reflect the best-available standard of care" for older adults. The same comparator the agency had previously green-lit.
Biotech Twitter (or whatever we're calling it now) erupted. The refusal felt inconsistent, almost like the left hand didn't know what the right hand had approved. But the plot twisted again: just about two weeks later, the FDA reversed course and accepted the application for review, setting a decision deadline of August 5, 2026.
If this sounds like a couple that keeps breaking up and getting back together, you're not wrong.
The Briefing Documents: Glass Half Full, Half Empty
Ahead of the June 18 VRBPAC meeting (that's the FDA's Vaccines and Related Biological Products Advisory Committee, essentially a panel of outside experts who vote on whether the benefits of a vaccine outweigh its risks), the FDA released its briefing documents. These reports are like a teacher's written evaluation before a student's oral exam: they frame the questions the panel will debate.
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The headline takeaway? "No major deficiencies were identified." The vaccine met all of its prespecified success criteria, including noninferiority, superiority, and even "super-superiority" against the standard-dose comparator.
In the pivotal trial, mRNA-1010 showed 26.6% higher efficacy than the standard-dose flu shot in preventing influenza illness among adults 50 and older. That held up across flu strains: roughly 30% better against H1N1, 22% better against H3N2, and 29% better against the B/Victoria lineage. In adults 65 and older specifically, efficacy clocked in at 27.4% above the comparator.
Jefferies analyst Andrew Tsai described the FDA's tone as "not harsh" and more balanced than expected, a characterization FierceBiotech highlighted in its coverage. Given the agency's earlier refusal-to-file drama, many on Wall Street had braced for a tougher review.
So far, so good. But the "mixed signals" part? The FDA also filled its briefing with a laundry list of concerns.
The Asterisks That Could Matter
The FDA flagged several "key uncertainties and evidence gaps" for the panel to chew on. Think of them as the fine print on an otherwise encouraging report card.
One season of data. All the efficacy evidence comes from a single flu season. Flu is notoriously unpredictable; strains shift year to year. The FDA wants to know: can we trust these results to hold up when a different variant dominates?
The comparator problem (again). For adults 65 and older, the standard of care in the U.S. isn't a regular flu shot. It's high-dose, adjuvanted, or recombinant vaccines like Sanofi's Fluzone High-Dose. Moderna compared mRNA-1010 against a standard-dose vaccine, not these premium options. That's like beating the JV team and claiming you're ready for the pros. Immunogenicity data (antibody levels) suggest mRNA-1010 can compete with high-dose vaccines, but direct clinical efficacy data against them simply don't exist yet.
Missing populations. The trial didn't include enough immunocompromised or very frail older adults to draw conclusions about them. The FDA notes this is "significant because these populations face the highest absolute risk" of severe flu complications. In other words, the people who need this vaccine most are the ones we know least about.
Influenza B remains a question mark. There weren't enough B/Victoria cases in the trial to clearly prove the vaccine protects against that strain. For a vaccine seeking a quadrivalent label, that's a notable gap.
No co-administration data. Adults often get their flu shot alongside COVID, RSV, or pneumococcal vaccines. Moderna hasn't studied those combinations yet, which matters for real-world vaccination programs.
Short safety follow-up. The safety database covers about six months in relatively healthy participants. No serious red flags emerged; the most common side effects were injection-site pain, fatigue, and headache. But reviewers noted the need for ongoing surveillance of rare events like myocarditis, which has been linked to other mRNA vaccines.
Why This Vote Matters Beyond Moderna
This isn't just about one company's flu shot. This is about whether mRNA technology earns its first approval for seasonal influenza, period.
Moderna is the front-runner by a comfortable margin. Pfizer-BioNTech and GSK are developing their own mRNA flu vaccines, but they're estimated to be one to two seasons behind. Sanofi, the current king of flu vaccines, actually scrapped its seasonal mRNA flu program entirely, betting the technology isn't mature enough to unseat its traditional products.
If mRNA-1010 (branded as mFlusiva) gets approved, Moderna would establish the first mRNA flu brand and start locking in pharmacy shelf space, physician familiarity, and government contracts before any competitor can catch up.
The stakes for Thursday's VRBPAC vote are high. The panel will consider two separate questions: do the benefits outweigh the risks for adults 50 to 64, and do they outweigh the risks for those 65 and older? The second question is where the tension lives, given the comparator controversy and the data gaps in frail, elderly populations.
The Bottom Line
The FDA's pre-meeting assessment reads like a contradictory performance review: "Great work, but also, we have concerns." No major deficiencies, yet a stack of evidence gaps that could give cautious panelists pause.
William Blair analyst Myles Minter has noted that mFlusiva could become a meaningful revenue driver starting in 2027 if approved, but he's keeping a Market Perform rating on Moderna.
For Moderna, the path forward likely involves some form of conditional approval with post-marketing studies, particularly a confirmatory trial in older and high-risk adults. That's a reasonable outcome, and one the company seems prepared for.
But after a year of regulatory whiplash (refused, then accepted, then praised, then questioned), nobody should be surprised if Thursday's vote comes with its own set of mixed signals. The only consistent thing about this process has been its inconsistency.
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