The FDA has told the makers of six flu vaccines that they need to add a notification to the label of their shots, warning of the risk of febrile seizures, which can occur after childhood vaccinations and typically last no more than a few minutes and are generally harmless.
The FDA sent the notifications to Sanofi, AstraZeneca, GSK and CSL Seqirus earlier this month. Each of the six vaccines has been on the market in the U.S. for at least 13 years, including Sanofi’s Fluzone, which was blessed by the agency in 1980. Other shots cited in the letters, which were sent on January 9, were AZ’s FluMist, GSK’s Fluarix and FluLaval, and CSL’s Afluria and Flucelvax.
In the notices (PDF) to the companies, the FDA wrote that two postmarketing observational studies conducted by the FDA's Biologics Effectiveness and Safety System (BEST) revealed an increased risk of febrile seizures on the first day following vaccination in children aged 6 months to 4 years.
The flu shots were assessed over two flu seasons, from 2023 to 2025, using three commercial insurance claim data sources, the U.S. regulator told the companies. Among children ages 6 months to 4 years, the risk of febrile seizures within a window of 0 to 1 days after vaccination was compared to the risk within a control window of 8 to 63 days after vaccination, the FDA said.
The FDA did not include full data in the notifications, but did say that information from one data partner indicated an estimated risk of 21 febrile seizure episodes for every 1 million quadrivalent vaccinations. For trivalent vaccinations from the same data partner, the estimated rate was 44 seizures for every 1 million shots, the FDA said.
A spokesperson for Sanofi said that of the billions of doses of Fluzone that have been administered, febrile seizures have occurred in a “limited subset of patients.” The company already includes information on febrile seizures in its current prescribing information for the shot.
“Sanofi maintains an ongoing commitment to robust safety surveillance and effectiveness monitoring, informed by clinical and postmarketing data,” the spokesperson wrote in an email. “The existing data continue to support the established safety and efficacy profile of Fluzone SD.”
A GSK spokesperson said that the company is reviewing the FDA's label update request.
“Our top priority is patient safety," the spokesperson added. "We are committed to monitoring and ensuring the safety of all our products, including our seasonal influenza vaccines, and we are confident in their safety and efficacy profile."
The FDA wants companies to use the following language on their labels: “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.”
The FDA has given the companies 30 days to agree to include the suggested label update, propose changes to the notice, or submit a rebuttal explaining why a change is not needed.
Earlier this month, the CDC removed broad recommendations for flu shots for children in a sweeping move in which the U.S. health agency upended the childhood immunization schedule, also eliminating recommendations for other vaccines that protect them from diseases including COVID-19, rotavirus, meningitis, hepatitis A and hepatitis B.
Additionally, immunization against respiratory syncytial virus (RSV)—the top cause for infant hospitalizations in the U.S., according to the CDC—is now limited to those at high risk and children whose mothers have not been vaccinated.
Vaccines have been under heightened scrutiny since Robert F. Kennedy Jr., a longtime anti-vaccine activist, took over at the Department of Health and Human Services. Last year, RFK Jr. removed all 17 former experts on the CDC’s Advisory Committee on Immunization Practices and replaced them with his own selections.
Sanofi's spokesperson warned that flu can pose “significant health risks, particularly for young children under 5 and individuals with specific pre-existing health conditions.” The American Academy of Pediatrics recommends annual flu vaccination for all pediatric patients aged 6 months and older who do not have a contraindication, the spokesperson said.
Infectious disease experts have warned that opting out of a universal recommendation for childhood flu shots could endanger those who are at a higher risk of complications.