FDA Actions Disrupt Flu Vaccine Industry with Safety Warnings and Moderna Rejection

The US Food and Drug Administration has notified six influenza vaccine manufacturers that they must add a warning about the risk of febrile (fever-related) seizures to their product labels. The notices were issued Jan. 9, 2026, under the FDA's authority to mandate safety labeling changes. The manufacturers include Sanofi, AstraZeneca, GlaxoSmithKline (GSK), and CSL Seqirus.

The FDA cited postmarketing safety data from the 2023–2024 and 2024–2025 flu seasons showing a statistically significant increase in febrile seizures within one day of vaccination among children ages six months through four years. The agency stated the analyses "suggest a causal relationship" and estimated 21.2 excess febrile seizure cases per million standard-dose quadrivalent flu vaccinations and 44.2 excess cases per million standard-dose trivalent flu vaccinations.

Febrile seizures are convulsions triggered by fever and typically occur in infants and young children. The FDA instructed manufacturers to include new label language noting that "In two separate postmarketing observational studies, an increased risk of febrile seizures was observed during the first day following vaccination with standard dose trivalent (2024–2025) and quadrivalent (2023–2024) influenza vaccines in children 6 months through 4 years of age." Manufacturers have 30 days to accept the revised language, propose alternative wording, or formally rebut the changes.

In a separate action, on Feb. 10, pharmaceutical company Moderna announced that the FDA Center for Biologics Evaluation and Research (CBER) had refused to review its application to market an investigational mRNA-based flu vaccine. Without approval from the agency, the investigational vaccine cannot be sold in the US.

The director of the CBER decided not to review the drug application over the objection of agency staff. In a letter sent to Moderna announcing the decision, the director criticized the design of a clinical trial testing the vaccine candidate. Yet Moderna says the letter is inconsistent with previous written communications that the firm received from CBER about study design and endpoints.

The CEO stated, "This decision by CBER, which did not identify any safety or efficacy concerns with our product, does not further our shared goal of enhancing America's leadership in developing innovative medicines." The clinical trial at issue, wherein Moderna compared its mRNA flu vaccine candidate to currently licensed vaccines, showed that the mRNA shot was more effective. The Moderna flu vaccine candidate has been accepted for review in the EU, Canada, and Australia.

Drug approval is the culmination of a years-long regulatory process, and historically the agency has worked closely with companies to define the sort of evidence that would demonstrate a drug or vaccine is safe and effective. A vaccinologist at the University of Arkansas for Medical Sciences and medical director of the nonprofit National Foundation for Infectious Disease stated, "It's very unusual for FDA to refuse to review a phase 3 trial they were involved in planning for."

The same expert expressed concern about the current situation: "I don't like to use the term chaos, but it seems apropos at this point. Are they going to allocate as many resources to vaccine development for the US market as they have in the past?"

In past years, the FDA's Vaccines and Related Biological Products Advisory Committee (VRBPAC) has met in February or March to forecast and select strains to vaccinate against in the following flu season. Last year, the agency canceled a planned VRBPAC meeting. The move prompted concerns among experts about whether manufacturers would have adequate time to make the year's shots. After a meeting closed to the public, FDA later announced recommended strains in March. In a Mar. 14 press release following the news, Sanofi wrote that it "had already begun its annual production of vaccines" against influenza, and that the strains selected by the FDA matched what Sanofi was already producing because they had also been chosen weeks earlier by the World Health Organization (WHO). WHO is set to make this year's announcement on Feb. 26.