On Aug. 5, 2026, the U.S. Food and Drug Administration (FDA) approved Moderna’s seasonal flu shot, mFLUSIVA (mRNA-1010), making it the first messenger RNA platform for seasonal influenza in the United States. The decision permits health care providers to use mRNA technology for annual influenza vaccination in eligible age groups.1
FDA regulators approved mFLUSIVA to prevent influenza virus subtypes A and B in adults 50 years of age and older. The agency structured the authorization into two distinct regulatory paths based on patient age.2
For adults aged 50 to 64, the FDA granted traditional, full approval after Phase 3 clinical trials evaluated the shot against standard-dose, egg-based influenza vaccines.3
For adults aged 65 and older, the FDA granted conditional clearance under its Accelerated Approval pathway. This conditional clearance for seniors is based on blood test results showing antibody levels rather than waiting for long-term study results proving it prevents actual influenza infections compared to existing high-dose influenza vaccines. As part of this approval, federal regulators require Moderna to conduct a post-marketing confirmatory trial to evaluate the shot’s clinical effectiveness in senior populations.4
Regulatory Refusal of Review and File Resubmission
Before securing regulatory clearance, Moderna received a Refusal-to-File letter from the FDA on Feb. 3, 2026, that halted the initial review of its application.5
The FDA issued a formal Refusal to File letter declining to review Moderna’s Biologics License Application (BLA) for its mRNA-based seasonal influenza vaccine, mRNA-1010, citing a failure to meet initial filing requirements rather than conducting a full scientific evaluation. The agency’s decision focused on the Phase 3 clinical trial design, determining that the study was not “adequate and well-controlled” because Moderna evaluated mRNA-1010 against GlaxoSmithKline’s standard-dose Fluarix Quadrivalent rather than a high-dose influenza vaccine, which is the CDC-recommended standard of care for adults aged 65 and older who face a higher risk of serious influenza complications.
Senior FDA officials, including FDA commissioner at the time, Marty Makary, MD, questioned this methodology, suggesting that using a standard-dose shot as the control arm rather than the best available high-dose option was questionable and potentially made the mRNA flu shot appear artificially more effective.
While the FDA defended its decision as a necessary measure to maintain strict approval benchmarks without preventing future resubmissions, Moderna publicly disputed the rejection, stating that agency officials had previously agreed during meetings dating back to April 2024 that comparing the mRNA shot to Fluarix was an acceptable study plan.6
After Moderna revealed that it had received a Refusal-to-File letter, the company quickly scheduled a Type A meeting with the FDA. During this meeting, the agency accepted Moderna’s revised regulatory plan based on patient age, filing for traditional, full approval of mRNA-1010 for adults aged 50 to 64, while seeking accelerated approval for those 65 and older.7
Adverse Effects and Clinical Warnings
Clinical evaluations involving over 40,000 trial participants evaluated local and systemic adverse effects associated with mFLUSIVA’s lipid nanoparticle delivery system.8
Pain at the injection site occurred in 68.7 percent of adults aged 50 to 64 and 62.9 percent of adults aged 65 and older. Additionally, up to 20.3 percent of recipients reported swelling or tenderness in underarm lymph nodes near the injection site. Systemic reactions reported across trial groups included fatigue at rates up to 48.1 percent, headaches up to 41.9 percent, muscle pain up to 40.6 percent, joint pain up to 31.5 percent, and chills up to 27.5 percent.9
The prescribing label lists specific adverse effects and severe clinical warnings. The FDA explicitly states that mFLUSIVA can trigger severe allergic reactions, including acute anaphylaxis. Clinics must maintain medical equipment, including epinephrine, available during administration.10
Regulators advise health care providers to evaluate potential risks before administering the shot to individuals with a history of Guillain-Barré syndrome, a neurological condition involving nerve damage and weakness following prior influeza vaccinations. Vasovagal syncope or fainting can occur immediately after injection, requiring clinic staff to take precautions against fall-related injuries.11
During the six-month postvaccination follow-up in Study 2 (median follow-up of 171 days, ranging from 1 to 207 days), serious adverse events were reported in 2.7 percent (41 out of 1,502) of participants receiving Moderna’s mRNA quadrivalent influenza shot and 2.6 percent (38 out of 1,489) of participants in the high-dose comparator group. According to Moderna, none of the reported serious adverse events were considered causally related to mFLUSIVA.12
If you would like to receive an e-mail notice of the most recent articles published in The Vaccine Reaction each week, click here.
Click here to view References:1 U.S. Food and Drug Administration. MFLUSIVA. Aug. 7, 2026.
2 FDA. MFLUSIVA Package Insert. Aug. 7, 2026.
3 Ibid.
4 FDA. BLA Clinical Review Memorandum. Aug. 6, 2026.
5 Parpia R. FDA Refuses to Review Moderna’s mRNA Influenza Shot License Application. The Vaccine Reaction Feb. 18, 2026.
6 Ibid.
7 McKenzie H. Moderna’s mRNA Flu Shot Application, Promising August Decision. BioSpace Feb. 18, 2026.
8 FDA. MFLUSIVA Package Insert. Aug. 7, 2026.
9 Ibid.
10 Ibid.
11 Ibid.
12 Ibid.
