U.S. regulators have refused to review Moderna Inc.’s novel mRNA flu vaccine, dealing a major blow to the company as it seeks more products beyond its COVID shot.

The U.S. Food and Drug Administration rejected the company’s application, which was submitted in December, saying the studies conducted in support of the application didn’t compare the experimental shot to the “best-available standard of care,” Moderna said in a statement.

The rejection letter signed by Vinay Prasad, who leads the FDA division that oversees vaccines, cited the lack of an “adequate and well-controlled” study for mRNA-1010. Moderna said the FDA’s letter was “inconsistent with previous written communications” and noted that its studies compared the new shot to existing, approved vaccines for influenza.

The letter didn’t identify any safety or efficacy concerns, Moderna said. The company has requested an urgent meeting to discuss the move with the agency.

Moderna shares have risen 42% so far this year as of Tuesday’s close. The company said it doesn’t expect the FDA’s decision to impact its 2026 financial guidance.

The rare refusal to file appears to reflect Prasad’s longstanding criticism of control arms in clinical trials. As an academic before he joined the FDA, he was often harshly critical of the treatments drug companies chose for comparison purposes when testing new drugs for cancer and other diseases. He noted that drug companies could make their new treatments look good by picking easy comparators. Now he’s putting those criticisms into practice as a top regulator at the FDA.

In particular, Moderna said the FDA criticized the choice of control groups in its main flu vaccine trial, noting that it used a standard-dose vaccine for comparison purposes. The study included people over 65 years old, a group for whom high-dose flu vaccines are often recommended.

Refusal-to-file letters are a rare way to reject a drug. The FDA historically uses the technique when an application is considered grossly inadequate, such as when it’s missing key parts. The move by Prasad to not even consider Moderna’s application signals a new aggressive phase to regulating drugs and forcing drugmakers to perform more rigorous studies.

A 2021 study published in JAMA Internal Medicine found that only 4% of drug applications received refusal-to-file letters. Among the 2,475 applications that the FDA received during the study time frame, just 98 received such letters.

After meeting with the FDA, Moderna can file the application again over protest if it chooses, the rejection letter says. One factor in Moderna’s favor is that while many doctors recommend the high-dose vaccine for older people, the standard dose is also approved for them.

It’s unclear whether Moderna will decide to run another study. Moderna’s chief executive officer told Bloomberg last month that it doesn’t plan to invest in new late-stage vaccine trials because of growing opposition to immunizations from U.S. officials.

Moderna has also applied for approval of its flu shot in Europe, Canada and Australia. The U.S. has been mired in a particularly bad flu season, with the current vaccines not well-matched to the circulating strain. Moderna says its mRNA technology can solve this problem because its vaccine can be more quickly updated.

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