Moderna, a leading biotechnology company, announced on Tuesday that the U.S. Food and Drug Administration’s (FDA) Center for Biologics Evaluation and Research (CBER) issued a Refusal-to-File (RTF) letter for its investigational mRNA-based influenza vaccine, mRNA-1010. This unexpected decision prevents the agency from initiating a substantive review of Moderna’s Biologics License Application (BLA), casting uncertainty on the future of the innovative vaccine in the U.S. market, despite its acceptance for review by regulatory bodies in the European Union, Canada, and Australia.
The core of the FDA’s refusal, as detailed by Moderna, hinges solely on the choice of comparator vaccine used in the Phase 3 clinical trial for mRNA-1010. The agency cited a lack of an "adequate and well-controlled" study with a comparator arm that "does not reflect the best-available standard of care," specifically for adults aged 65 and older. Moderna’s trial utilized Fluarix, a standard-dose seasonal influenza vaccine, as the comparator. The FDA indicated that for the elderly demographic, the trial should have instead compared mRNA-1010 against a high-dose vaccine, which it now designates as the "best-available standard of care" for this vulnerable age group.
Moderna has vehemently contested this decision, highlighting that neither existing relevant regulations nor the FDA’s own guidance documents contain any explicit reference to the requirement of using a "best-available standard of care" comparator. Furthermore, the company emphasized that the RTF letter did not raise any specific concerns regarding the safety or efficacy profile of mRNA-1010. This rejection comes as a significant surprise to Moderna, as the company claims it contradicts feedback received during extensive pre-Phase 3 and pre-submission consultations with CBER. Moderna has formally requested a meeting with the FDA to seek clarification and understand the path forward.
The Regulatory Impasse: A Deeper Look at the Comparator Dispute
The dispute over the comparator vaccine is central to the FDA’s RTF decision. For influenza vaccines, particularly in older adults, the distinction between standard-dose and high-dose options is clinically significant. Older adults generally exhibit a diminished immune response to standard-dose vaccines due to immunosenescence, the gradual deterioration of the immune system with age. To address this, high-dose influenza vaccines, which contain four times the antigen of standard-dose vaccines, were developed and approved to elicit a stronger immune response and provide enhanced protection in individuals 65 years and older.
The FDA’s stance that a high-dose vaccine constitutes the "best-available standard of care" for this demographic is based on clinical evidence demonstrating superior efficacy in this specific population. However, Moderna’s argument is rooted in the regulatory precedent and the consultative process. In April 2024, Moderna submitted its Phase 3 study protocol to CBER for review. While CBER did recommend considering a high-dose vaccine for the comparator arm in adults over 65, Moderna asserts that the agency did not raise any objections or issue clinical hold comments concerning the overall adequacy of the trial design at that time. Moreover, following the completion of the trial, Moderna held a pre-submission meeting with CBER, during which, according to the company, no indication was given that the application would be refused for review. This perceived shift in the FDA’s position, from a recommendation to an absolute requirement for filing, forms the crux of Moderna’s appeal.
mRNA-1010’s Efficacy Profile and Global Acceptance
Despite the U.S. regulatory setback, mRNA-1010 has demonstrated promising results in its Phase 3 trials. The vaccine showed a 26.6% higher relative vaccine efficacy compared to the standard-dose comparator, Fluarix. This efficacy was robust across individual strains, registering 29.6% for A/H1N1, 22.2% for A/H3N2, and 29.1% for B/Victoria. Crucially, for the age group at the center of the FDA’s concern, adults 65 and older, mRNA-1010 exhibited 27.4% relative efficacy. These efficacy rates are considered significant in the context of annual influenza vaccine development, where incremental improvements can have substantial public health benefits.
The global regulatory landscape for mRNA-1010 also stands in stark contrast to the FDA’s decision. The vaccine’s application has been submitted and accepted for review in the European Union, Canada, and Australia. This international acceptance suggests that regulatory bodies in these regions found the clinical trial design and submitted data package sufficient for substantive evaluation, further underscoring the unusual nature of the FDA’s RTF.
Rarity of Refusal-to-File Letters and Alleged Internal Disagreement
Refusal-to-File letters are an uncommon occurrence in the FDA’s drug and biologic application process. A 2021 study analyzing 2,475 applications found that only approximately 4% received an RTF letter. This rarity suggests that such decisions are typically reserved for applications with fundamental and unresolvable deficiencies in their completeness or quality, rather than a disagreement over the nuances of a comparator arm after extensive prior consultation.
Adding another layer of complexity to the situation, a report by STAT, a prominent health and medicine news outlet, revealed that CBER Director Vinay Prasad allegedly overruled his own agency’s career scientists in making this decision. According to STAT, three agency officials familiar with the matter stated that the team of career scientists was prepared to review Moderna’s application. Furthermore, David Kaslow, the head of the vaccine office, reportedly authored a detailed memo advocating for the FDA to proceed with the review. If true, this internal disagreement highlights a potential departure from standard scientific consensus within the agency, raising questions about the final decision-making process.
A Broader Context: Vaccine Policy Shifts Under the Trump-Kennedy Administration
This specific RTF decision by the FDA does not occur in a vacuum but is situated within a discernible pattern of policy shifts and actions concerning vaccines under the current Trump administration. Robert F. Kennedy Jr., a long-time vaccine skeptic, assumed the role of Secretary of Health and Human Services (HHS) in February 2025. Since his appointment, his tenure has been marked by significant changes and controversial directives impacting U.S. vaccine policy and research.
Just months after Kennedy took office, in August 2025, HHS canceled $500 million in funding from the Biomedical Advanced Research and Development Authority (BARDA) specifically earmarked for mRNA vaccine development. BARDA plays a critical role in supporting the development of medical countermeasures, including vaccines, for public health emergencies. The cancellation of this funding signaled a clear shift away from prioritizing mRNA technology, which had proven pivotal in the rapid development of COVID-19 vaccines.
Further illustrating this trend, in December 2025, Children’s Health Defense (CHD), an organization founded by Robert F. Kennedy Jr. himself, formally petitioned the FDA to revoke the licenses for Moderna and Pfizer’s widely used COVID-19 mRNA vaccines. While CHD is a separate entity from HHS, the timing and nature of this petition, coupled with Kennedy’s leadership at HHS, contributed to an environment of increased scrutiny and skepticism towards mRNA vaccine technology from within governmental and allied circles.
CBER Director Vinay Prasad, who signed the RTF letter to Moderna, has also been a vocal critic of mRNA vaccines and certain aspects of public health policy related to them. His appointment to such a critical regulatory position, combined with his public stance, has been a point of concern for many in the scientific and public health communities who champion vaccine innovation and broad access.
Implications for Moderna, mRNA Technology, and Public Health
The FDA’s refusal to file Moderna’s mRNA flu vaccine application carries several significant implications:
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Impact on Moderna: For Moderna, this decision represents a considerable setback for its influenza vaccine program in the lucrative U.S. market. While mRNA-1010 is progressing internationally, the U.S. remains a key market. Delays mean lost potential revenue and a prolonged timeline for bringing this innovation to American patients. It also creates uncertainty regarding the company’s future interactions with the FDA for other mRNA vaccine candidates.
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Future of mRNA Flu Vaccines: If approved, mRNA-1010 would have been the first mRNA-based influenza vaccine. The RTF could dampen enthusiasm and investment in developing next-generation mRNA vaccines for influenza and other infectious diseases in the U.S. The promise of mRNA technology lies in its speed of development, adaptability to new strains, and potential for enhanced efficacy. Hindering its path to market could slow innovation in a field where annual improvements are crucial.
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Public Health Consequences: Influenza remains a significant public health burden, causing millions of illnesses, hundreds of thousands of hospitalizations, and tens of thousands of deaths annually in the U.S. New, more efficacious vaccines are continually sought to reduce this burden. The over-65 population is particularly vulnerable, and improved vaccine options for them are critical. Blocking a potentially more effective vaccine, especially one showing strong efficacy in older adults, could limit choices for vulnerable populations and potentially impact public health outcomes.
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Regulatory Integrity and Precedent: The alleged overruling of career FDA scientists by CBER leadership, coupled with the perceived inconsistency with prior consultations, raises concerns about the scientific integrity and predictability of the regulatory review process. An RTF letter, particularly one based on a criterion not explicitly outlined in guidance documents and seemingly introduced late in the process, could set a troubling precedent for future drug and biologic development. It could also deter pharmaceutical companies from investing in innovative, complex trial designs if the goalposts for approval appear to shift without clear regulatory basis.
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Political Influence on Science: The broader context of the Trump-Kennedy administration’s stance on vaccines inevitably intertwines with this regulatory decision. Critics argue that the decision reflects a politicization of scientific review, where skepticism towards specific technologies or a broader anti-vaccine sentiment may be influencing regulatory outcomes. This perception, whether accurate or not, risks eroding public trust in both regulatory bodies and scientific innovation.
Looking Ahead: Moderna’s Path and the Broader Vaccine Landscape
Moderna’s immediate next step is its requested meeting with the FDA. This meeting will be crucial for understanding the specific regulatory hurdles and whether there is a viable path forward for mRNA-1010 in the U.S. This could involve conducting additional clinical trials with a high-dose comparator, though such an endeavor would be time-consuming and costly.
Meanwhile, the global acceptance of mRNA-1010’s application in the EU, Canada, and Australia may allow Moderna to launch the vaccine in these markets if approved, providing real-world data that could eventually support a resubmission to the FDA or influence future U.S. regulatory discussions.
The situation surrounding Moderna’s mRNA flu vaccine highlights a complex interplay between scientific innovation, evolving regulatory standards, and the political climate. As the debate unfolds, the scientific community, public health advocates, and patients will closely watch how this decision impacts not only Moderna’s specific vaccine but also the broader trajectory of mRNA technology and vaccine policy in the United States.
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