Study finds elevated febrile seizure risk after COVID-19 vaccination

A peer-reviewed study funded and conducted by the U.S. Food and Drug Administration (FDA) has found that young children who received Moderna’s COVID-19 vaccine faced a higher risk of febrile seizures within one day of vaccination compared with later periods after the shot.

The study, published on February 4 in the medical journal Vaccine, reported that among children aged 2 to 4 years, the risk of a febrile seizure in the first 24 hours after receiving Moderna’s vaccine was about two-and-a-half times higher than during the comparison window of 8 to 63 days post-vaccination.

The research was carried out under the FDA’s Biologics Effectiveness and Safety (BEST) System, which monitors vaccine safety using large healthcare databases.

However, the publication included a disclaimer noting that the findings “should not be construed to represent FDA’s views or policies.”

Karl Jablonowski, Ph.D., senior research scientist at Children’s Health Defense (CHD), described the publication as notable, saying it was unusual for an FDA-led study highlighting a vaccine safety concern to appear in a major peer-reviewed journal. “That likely wouldn’t have happened a few years ago,” he said.

The FDA did not respond to questions about whether it publicly announced the febrile seizure safety signal when the study first appeared as a preprint in March 2024.

Separately, the agency asked manufacturers of several influenza vaccines last month to add warnings about the risk of febrile seizures in young children.

The Centers for Disease Control and Prevention (CDC) acknowledges on its COVID-19 vaccine safety webpage that febrile seizures have been reported following Moderna vaccination, including during the company’s clinical trials.

At the time the data were collected, both Moderna and Pfizer COVID-19 vaccines were authorized for emergency use in children under 5. Since then, Pfizer’s vaccine has been licensed only for children aged 5 and older, while Moderna’s vaccine is licensed for infants and children aged 6 months and up.

In October 2025, the CDC updated its COVID-19 vaccination guidance for children aged 6 months and older, shifting away from routine recommendations and instead advising “individual-based decision-making.”

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Daniel O’Connor, founder and CEO of TrialSite News, which first reported on the FDA study, said the findings provide parents with data to help weigh potential benefits and risks in the absence of a blanket recommendation. Jablonowski, however, disputed that interpretation, arguing that the study’s 63-day follow-up period was too short to assess possible long-term neurological outcomes.

“Neurological harm would likely only be evident throughout the child’s development and not confined to the 63 observation days,” he said.

The study’s corresponding author did not respond to a request for comment by the publication deadline.

Febrile seizures are convulsions associated with fever, most commonly occurring in children between 6 months and 5 years of age when body temperature rises above 100.4°F. They are often triggered by infections linked to common childhood illnesses.

Most febrile seizures last less than 15 minutes and are generally considered not life-threatening. Medical outlets such as MedPage Today note that they typically do not cause permanent harm or lasting effects. However, Brian Hooker, Ph.D., CHD’s chief scientific officer, challenged that view, stating, “Any seizure is bad, period.”

Hooker said some studies suggest that even so-called “mild” febrile seizures may double a child’s risk of developing epilepsy, while prolonged or “complex” febrile seizures lasting more than 15 minutes could raise that risk up to tenfold.

Jablonowski also cited reviews published in 2023 and 2024 that concluded febrile seizures during brain development may be associated with long-term neurological consequences in a subset of children.

The researchers analyzed insurance claims data from mid-2022 to mid-2023 for children aged 2 to 5, drawing from three major insurers—Carelon Research, CVS Health and Optum—which together represent about 17% of U.S. pediatric claims.

They compared the incidence of febrile seizures occurring within 0–1 days after vaccination with those reported 8–63 days later.

Moderna’s vaccine showed a statistically significant increase in febrile seizures in the first day after vaccination, while Pfizer’s vaccine showed an increase that did not reach statistical significance.

Jablonowski suggested a biological explanation for the difference between the two vaccines, noting that Moderna’s pediatric dose contains 25 micrograms of mRNA, compared with 3 micrograms in Pfizer’s dose.

He also claimed Moderna’s product contained higher levels of residual DNA, which he said could activate the innate immune system more strongly, potentially increasing fever-related reactions.