The U.S. Food and Drug Administration (FDA) has opted not to review Moderna’s licensing application for a new mRNA influenza vaccine, a move officials say reflects the agency’s commitment to ensuring vaccine approvals are based on robust and well-controlled clinical data.

The decision comes after Moderna submitted trial results comparing its experimental vaccine against existing standard flu shots. While the company reported strong antibody responses and no safety concerns, the FDA determined the studies did not fully reflect the “best-available standard of care” for certain populations, particularly high-risk adults, according to a letter from Vinay Prasad, director of the Center for Biologics Evaluation and Research (CBER).

In adults under 65, Moderna compared the new vaccine against standard flu shots, rather than high-dose formulations typically recommended for older adults. FDA officials said this trial design made it difficult to fully assess the vaccine’s comparative effectiveness across all recommended populations. For adults 65 and older, Moderna did conduct high-dose comparisons, which the agency noted as positive steps, but additional information is required to ensure the vaccine meets rigorous standards for safety and efficacy.

“By requesting more comprehensive data, the FDA is maintaining the integrity of its review process and ensuring that any new vaccines provide clear, measurable benefits over existing options,” said public health experts familiar with regulatory standards.

mRNA vaccines, which allow rapid updates to respond to evolving viral strains, have shown promise in improving protection against influenza. FDA officials emphasized that the refusal-to-file letter does not reflect any safety or efficacy concerns, but rather a need for clarity in how the new shot performs compared with the highest standards of current care.

Stéphane Bancel, Moderna CEO, said in a statement, “The decision should not be seen as a question of safety. We look forward to providing additional data to support a comprehensive review.” Moderna has requested a meeting with the FDA to discuss next steps.

The licensing application is still under review in other countries, including the European Union, Canada, and Australia. In the U.S., the FDA’s approach underscores its commitment to careful, science-based evaluation before approving vaccines for broad public use, ensuring that any newly authorized products deliver meaningful benefits for all Americans.