The U.S. Food and Drug Administration (FDA) notified six influenza vaccine manufacturers on Jan. 9, 2026 that they must add a warning about the risk of febrile (fever) seizures to their product information labels, citing newly identified postmarketing safety data. The notices, issued under the FDA’s statutory authority to mandate safety labeling changes, were sent to Sanofi, AstraZeneca, GlaxoSmithKline (GSK), and CSL Seqirus.1
According to the FDA, observational analyses conducted during the 2023–2024 and 2024–2025 influenza seasons identified a statistically significant increase in seizures caused by fevers occurring within one day of vaccination among children aged six months through four years, prompting the agency to conclude the findings constitute “new safety information” requiring disclosure in product labeling. In its notification letter, the FDA stated that the results of the analyses “suggest a causal relationship”—language the agency typically avoids unless compelled by evidence.2
The data suggested an “estimated attributable risk of 21.2 excess febrile seizure episodes per million standard-dose quadrivalent influenza vaccinations” and “an attributable risk of 44.2 excess febrile seizure episodes per million standard doses of trivalent [influenza] vaccinations.”
Risk of Febrile Seizures May Increase After Childhood Vaccinations
The Mayo Clinic website states that in some infants and young children fevers can cause convulsions (seizures). Doctors generally describe “simple” febrile seizures as associated with fevers over 100 F, short-lived and typically harmless, although “complex” febrile seizures can last longer than 15 minutes and occur more than once in a 24-hour period.3
It has been acknowledged in the medical literature that convulsions can include an increased risk of future epilepsy (uncontrolled seizure disorder).4 5 Febrile seizures have also been associated with a spectrum of brain dysfunction and rarely, severe brain injury or death.6 7
Bioengineer Brian Hooker, PhD said he disagrees with febrile seizures being characterized as harmless. He said:
Any seizure is bad, period. Mild’ febrile seizures can double a child’s chance of an epilepsy diagnosis and ‘complex’ febrile seizures—lasting more than 15 minutes —can increase that risk up to 10 times.8
On the topic of febrile seizures and childhood vaccines, Mayo Clinic states:
The risk of febrile seizures may increase after some childhood vaccinations. These include the diphtheria, tetanus and pertussis vaccine and the measles-mumps-rubella vaccine. A child can develop a low-grade fever after a vaccination. The fever, not the vaccine, causes the seizure.9
The U.S. Centers for Disease Control and Prevention (CDC) acknowledges that some vaccines are associated with a higher risk of febrile seizures than others, particularly in infants and young children, and that the risk increases when certain vaccines—such as the influenza, pneumococcal (PCV13), and DTaP vaccines—are administered during the same visit.
The agency also notes that measles-containing vaccines, especially the MMRV (measles, mumps, rubella, and varicella) combination, are associated with a higher risk of febrile seizures than their single-component counterparts, and that these events tend to occur within specific post-vaccination time windows. Still, CDC guidance emphasizes that the overall risk is small, that febrile seizures are typically short-lived and without lasting harm, and that vaccination should continue according to the recommended schedule.10
Hellbound and Down 