The Food and Drug Administration has refused to review Moderna's application for an mRNA flu vaccine, the company revealed Tuesday. The decision blindsided the high-profile vaccine maker, which had spent hundreds of millions of dollars on a Phase 3 trial involving nearly 41,000 participants. The trial found that Moderna's mRNA-1010 was superior to licensed standard-dose influenza vaccines.

Moderna said the FDA had reviewed and accepted its trial design on at least two occasions before the company applied for approval. The agency's rejection centered on the comparator vaccine used in the trial, which Moderna noted has been used in previous flu vaccine trials that earned approval. In a letter dated February 3, the FDA's top vaccine regulator under the Trump administration told Moderna the trial was not "adequate and well-controlled" because the comparator did not reflect the "best-available standard of care."

Moderna CEO Stéphane Bancel called the decision controversial, noting it did not identify any safety or efficacy concerns with the product. The company has requested a meeting with the FDA to understand the basis for the refusal and determine the path forward. Moderna emphasized that mRNA-1010 has already been accepted for review in the European Union, Canada, and Australia.