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The US Food and Drug Administration (FDA) has refused to review Moderna's application for its new mRNA-based flu vaccine, dealing a major blow to the company's plans.

Moderna announced the decision Tuesday, and its stock fell 8% in extended trading, Reuters reported.

In a "refuse-to-file" letter dated February 3, the FDA said the company's application did not include an "adequate and well-controlled" study.

The agency said the control group in Moderna's trial did not reflect the "best-available standard of care in the United States at the time of the study." The FDA did not raise concerns about the vaccine's safety or how well it works.

Moderna said it used a standard-dose flu vaccine called Fluarix as a comparison in a 40,700-person clinical trial.

The company said the FDA had agreed to that plan in April 2024. It also included data comparing its vaccine to a high-dose flu shot for people 65 and older, as suggested by the agency.

"The complete stunner here is at no point in any of this did anybody say that it was not adequate," Moderna President Dr. Stephen Hoge told, referring to the trial design.

CEO Stéphane Bancel added that the decision "did not identify any safety or efficacy concerns with our product" and said it does not support the shared goal of advancing innovative medicines.

The FDA's Center for Biologics Evaluation and Research (CBER), which oversees vaccines, signed the letter.

The US Department of Health and Human Services said the FDA generally does not comment on communications with companies. Refuse-to-file letters are rare. A 2021 study found that only 4% of nearly 2,500 applications received such letters.

Moderna Says mRNA Flu Shot 27% More Effective

Moderna's phase 3 trial results, reported in June, showed the mRNA flu shot was about 27% more effective than Fluarix in adults 50 and older.

The company believes mRNA technology could make flu vaccines more effective and faster to produce.

Unlike traditional vaccines, which are planned months in advance, mRNA shots can be designed and made more quickly. That could allow better matching to flu strains circulating each season.

According to CNN, the setback comes as federal health officials reassess vaccine policies and as support for mRNA research has been reduced. Last year, the government canceled 22 mRNA-related projects worth about $500 million.

Moderna has requested a meeting with the FDA to discuss next steps.

Meanwhile, its flu vaccine and combination flu-COVID shot are moving forward in other countries. "We do expect that our first approval will be ... probably in Europe, and this year," Hoge said.

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