The US Food and Drug Administration (FDA) will not review Moderna’s application for a new mRNA flu vaccine. This move comes despite the FDA earlier giving the green light to Moderna’s project. The FDA’s refusal is based on Moderna’s clinical trials comparing the new vaccine with standard flu shots, not high-risk flu vaccines, which the FDA said “does not reflect the best-available standard of care.” Moderna compared their vaccine to a high-dose flu shot for those over 65 and standard vaccines for under 65, but the FDA found this approach inadequate. The new mRNA vaccine showed better antibody responses and no safety issues in trials. Moderna also has another trial comparing their vaccine with standard shots in adults over 50. However, the FDA issued a refusal-to-file letter, meaning it won’t consider the licensing application until more information is provided. The letter did not specify what information is needed. Notably, the letter was signed by the director of the Center for Biologics Evaluation and Research, a rare move. Experts like UC Law professor Dorit Reiss say this decision discourages vaccine makers and creates uncertainty. Moderna’s CEO Stéphane Bancel called the refusal “controversial” since the new vaccine used an FDA-approved comparator and trial plans were agreed on beforehand. The FDA may be revising rules for annual flu vaccines, potentially making approvals harder. This decision follows recent US health policy changes, such as limiting flu vaccine recommendations for children without public consultation. Moderna is seeking a meeting with the FDA to discuss the refusal. Meanwhile, the vaccine application is still under review in the EU, Canada, and Australia.