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
“Harmless” Febrile Seizures Can Have Long-Term Health Consequences
A 2023 scientific review published in Frontiers in Cell and Developmental Biology found evidence that febrile seizures occurring during early brain development may have long-term consequences later in life.11 According to the review, these outcomes can include conditions such as attention-deficit/hyperactivity disorder (ADHD), epilepsy, and cognitive decline in adolescence and adulthood.
For decades, parents have been told by the American Academy of Pediatrics (AAP) and doctors administering vaccines to children that febrile seizures are short-lived and harmless.12 However, the 2023 study provided more evidence that seizures occurring while the brains of infants and children are still developing can cause brain dysfunction that does not become fully apparent until years later.8
Dr. Hooker argues that pharmaceutical companies routinely minimize and normalize vaccine-related adverse events while failing to adequately consider how seizures may disproportionately affect certain populations, including children with autism. “Too many children are being harmed,” he said. “The rate of seizures in autistic people can be as high as 20 percent—with the damage swept under the rug.”8
Flu Shot Label Update Follows Years of Denial About Febrile Seizure Risk
The FDA’s decision follows years of messaging by public health officials and doctors administering vaccines that characterized vaccine-associated febrile seizures as rare and typically harmless. In 2024, a study examining febrile seizures following COVID-19 vaccination in young children confirmed a short-term increase in risk but was widely framed as clinically insignificant, with critics of the study’s conclusions characterized as exaggerating or misinterpreting the findings.
Commentary surrounding the 2024 study emphasized that febrile seizures are “usually harmless,” classified them as “rare” in absolute terms, and an expected consequence of fever rather than the vaccine itself. Often when parents have reported their children were having seizures after vaccination, those reports from parents were dismissed as “coincidental” rather than causal. Yet information published in the medical literature and from institutions such as the Mayo Clinic acknowledges that febrile seizures can and do occur after childhood vaccinations.10
Like many pharmaceutical product warning label updates that occur years or even decades after products are licensed and approved, the febrile seizure risk following influenza vaccinations only became apparent after widespread real-world use and reports of seizures following administration of flu shots to infants and children. A number of adverse events with permanent adverse health outcomes are not detected (or are dismissed by investigators as “coincidental”) during pre-licensure clinical trials of new vaccines, with trial participants often monitored for only a few days or weeks, a limitation critics argue makes it easier for relatively rare but serious harms to remain unnoticed until after widespread use.
The FDA’s citing of a causal relationship between febrile seizures and influenza vaccines requiring a new warning label for flu shots underscores that post-marketing safety signals, even when labeled “rare” or considered to be “coincidental,” still warrant public disclosure.
The FDA has instructed influenza vaccine manufacturers to include 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.9
Manufacturers have 30 days to either accept the revised language, propose alternative wording, or submit a formal rebuttal.2
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