FDA Issues Refusal-to-File Letter for Moderna's mRNA Flu Vaccine Over Trial Design Dispute

The FDA's Center for Biologics Evaluation and Research issued a refusal-to-file letter to Moderna for its investigational influenza vaccine, mRNA-1010, following concerns over the Phase 3 trial design. The director of CBER reportedly criticized the control arm, arguing it did not reflect the "best-available standard of care."

At issue is whether Moderna's Phase 3 trial design met current regulatory expectations. Moderna counters that its comparator—an FDA-approved influenza vaccine—was previously discussed and aligned with CBER before trial initiation. The company further notes that no objections were raised at trial launch or during pre-submission meetings, and that additional analyses requested in August 2025 were provided.

Internal dissent reportedly existed within the FDA. Review staff disagreement included a memo from the Office of Vaccines Research and Review cautioning against the refusal-to-file. HHS has acknowledged "a diverse set of conclusions" within the review team.

Moderna's public statements highlight potential inconsistency in FDA feedback across leadership transitions. The refusal-to-file may affect Moderna's broader influenza and combination vaccine strategy, including mRNA-1083.

The episode raises questions about whether the refusal-to-file represents a technical judgment about trial rigor, or evidence of a shifting evidentiary bar under new CBER leadership. Abrupt interpretive changes—especially after prior written alignment—can erode sponsor confidence and fuel perceptions of politicization. Consistent regulatory signaling remains foundational to public trust, industry planning, and scientific credibility.