Moderna’s once-rebuffed mRNA flu shot to face scrutiny from FDA adcomm

FDA Adcomm to Review Moderna's Rebuffed mRNA Flu Shot

The FDA will convene its Vaccines and Related Biological Products Advisory Committee on June 18, 2026, to assess Moderna’s investigational mRNA flu vaccine, MFLUSIVA. This meeting follows an initial refusal-to-file letter in February, which cited concerns that the pivotal trial's control group did not meet the best standard of care. After a subsequent compromise, the FDA accepted an amended application. Moderna is now seeking regular approval for adults aged 50-64 years and accelerated approval for those 65 years and up, with a final decision anticipated in August. The adcomm will discuss the vaccine's safety and efficacy for preventing influenza A and B in patients 50 years and older.

• In early February, the FDA issued a refusal-to-file letter for Moderna's flu vaccine application, contending that the pivotal trial's control group "does not reflect the best-available standard of care" and failed to adhere to 2024 FDA guidance requiring comparison against a CDC-recommended flu vaccine. This decision generated significant industry blowback and internal scrutiny within the Department of Health and Human Services.
• Following the initial rejection, Moderna and the FDA reached a compromise, leading the regulator to accept an amended application for review. This revised submission specifically seeks regular approval for use in adults aged 50 to 64 years and accelerated approval for individuals 65 years and older, with a final regulatory decision expected in August.
• The controversy surrounding Moderna's application and the subsequent investigation into former Center for Biologics Evaluation and Research (CBER) Director Vinay Prasad's professional conduct led to his departure. This event triggered a series of high-level reassignments and resignations at the FDA, including former Commissioner Marty Makary and other key CBER and CDER officials, highlighting significant leadership instability within the agency.

Moderna's MFLUSIVA: Navigating the FDA's Standard of Care Scrutiny

Recent clinical evidence reveals mixed results for investigational influenza therapies when compared to standard-of-care treatments. The most comprehensive recent data comes from pimodivir studies, where the investigational polymerase acidic protein inhibitor was tested in combination with standard-of-care (primarily oseltamivir). In hospitalized patients with influenza A, pimodivir at 600 mg twice daily for 5 days plus standard-of-care showed no clinical benefit over placebo plus standard-of-care on the Hospital Recovery Scale at Day 6. However, in high-risk outpatients, the combination demonstrated a statistically significant reduction in median time to resolution of influenza symptoms compared to placebo plus standard-of-care (92.6 hours versus 105.1 hours, P = 0.0216).

Current World Health Organization guidelines from 2025 provide conditional recommendations that reflect the evolving landscape of influenza treatment options. Baloxavir marboxil, a cap-dependent endonuclease inhibitor, receives conditional recommendation for non-severe influenza only when risk of severe illness is high, while antivirals are not recommended for low-risk cases. For severe influenza, oseltamivir remains the conditionally recommended standard, while peramivir and zanamivir are not recommended. The guidelines notably exclude macrolide antibiotics (absent co-infection), mTOR inhibitors, plasma therapy, and corticosteroids from recommended treatments for severe influenza.

Historical neuraminidase inhibitor data from controlled studies demonstrates that established therapies like zanamivir (10 mg inhaled twice daily) and oseltamivir (75 mg orally twice daily) consistently reduce systemic symptoms by 0.7-1.5 days, with greater efficacy (1.5-2.0 days reduction) observed in proven influenza cases and when treatment is initiated within 30 hours of symptom onset. The challenge for investigational therapies lies in demonstrating superior efficacy to these established benchmarks, particularly given concerns about antiviral resistance development and the recognition that combination approaches targeting both viral replication and immune response modulation may be necessary for severe infections, though such strategies require validation in carefully designed clinical trials.

Moderna's mRNA Flu Vaccine: A Platform's Pivotal Test

Moderna's journey to bring its mRNA flu vaccine, MFLUSIVA, before the FDA Advisory Committee is more than just a product review; it's a bellwether for the future of mRNA technology in infectious disease prevention. The potential approval of MFLUSIVA would represent a significant expansion of the mRNA platform beyond its groundbreaking success in COVID-19, demonstrating its versatility and capacity to address other major public health challenges like seasonal influenza. For older adults, a demographic particularly vulnerable to severe flu outcomes and often experiencing diminished immune responses to conventional vaccines, an mRNA-based option could offer a much-needed boost in protection. Studies have indicated that mRNA COVID-19 vaccines demonstrated high efficacy and immunogenicity even in older populations, suggesting a similar potential for MFLUSIVA.

However, the path to approval is not without its hurdles. The FDA's initial refusal-to-file letter, citing concerns about the control group's standard of care, highlights the rigorous regulatory environment and the need for robust comparative data. The upcoming AdComm will undoubtedly delve into the trial's design and the vaccine's safety and efficacy data, setting crucial precedents for how future mRNA vaccines for non-COVID indications will be evaluated. While mRNA COVID-19 vaccines have shown strong safety signals, rare adverse events such as myocarditis and allergic reactions have been reported, and the AdComm will carefully consider MFLUSIVA's profile in the context of these known platform-related considerations. Furthermore, the broader landscape of vaccine confidence, which saw significant challenges during the COVID-19 pandemic, could influence uptake of a novel mRNA flu vaccine. Overcoming potential public hesitancy will require clear communication and robust real-world data. Ultimately, the outcome of this AdComm will not only shape Moderna's trajectory in the flu market but also provide invaluable insights into the regulatory and public acceptance pathways for the next generation of mRNA-based therapeutics.