Why the FDA Really Rejected Moderna's Flu Vaccine

What happens when 40,000 clinical trial participants aren't enough? Moderna just found out. The FDA sent the biotech giant a "refusal-to-file" letter for its mRNA flu vaccine—a decision so unusual that it's raising questions about whether science or politics is driving American vaccine policy.

The Unprecedented Rejection

In December 2025, Moderna submitted its application for an mRNA-based flu vaccine targeting adults 50 and older. The vaccine had successfully completed clinical trials involving more than 40,000 participants—a massive study by any standard.

Yet on February 3, 2026, the FDA issued a refusal-to-file letter, claiming the application was "incomplete." This type of rejection is extraordinarily rare because companies work closely with the FDA from early development stages to ensure their applications meet requirements.

What makes this even more striking? Drug regulators in Canada, Europe, and Australia all accepted Moderna's application for review. The FDA stands alone in its refusal.

Internal Conflict Exposed

FDA documents reveal a troubling internal divide. David Kaslow, director of the Office of Vaccines Research and Review, wrote a memo recommending the application be reviewed. But his superior, Vinay Prasad, who heads the center overseeing vaccine research, overruled the decision.

Directors rarely override their scientific staff, especially on vaccine matters. Yet this marks at least the fourth time Prasad has done so since his 2025 appointment—a pattern that's raising eyebrows among FDA veterans.

The FDA's Shifting Standards

The FDA claimed Moderna failed to conduct an "adequate and well-controlled" study because it didn't compare patients to "the best-available standard of care." But here's the problem: that phrase appears nowhere in FDA law or vaccine guidance.

More troubling still, the FDA had previously approved Moderna's trial design in 2024, explicitly calling the standard-dose flu vaccine comparison "acceptable." Now, suddenly, it wasn't good enough.

As vaccine policy expert Ana Santos Rutschman notes, this decision could meet the legal standard for being "arbitrary and capricious"—grounds for court invalidation, though that process would take years.

Kennedy's Vaccine Skepticism in Action

This rejection fits a broader pattern under Health and Human Services Secretary Robert F. Kennedy Jr., who has expressed particular skepticism toward mRNA vaccines despite providing no credible safety data.

The timeline tells the story:

• April 2025: Kennedy announces new vaccines will require additional clinical trials
• November 2025: Prasad releases a memo claiming mRNA COVID vaccines killed children (without evidence)
• February 2026: FDA refuses to review Moderna's flu vaccine

The escalation is clear—and concerning.

Industry-Wide Implications

The practical consequences extend far beyond one vaccine. Pharmaceutical companies now face unprecedented uncertainty about FDA requirements that could change without warning or legal basis.

Moderna's CEO already announced in January 2026 that the company was scaling back vaccine development—even before this flu vaccine rejection. Other manufacturers are likely reassessing their own vaccine portfolios.

The message is chilling: invest billions in vaccine development at your own risk.

A Global Divergence

Perhaps most concerning is how this positions the United States relative to other major economies. While Canada, Europe, and Australia move forward with reviewing promising vaccine technologies, America appears to be stepping backward.

This isn't just about one flu vaccine—it's about America's role in global health innovation and pandemic preparedness.