Children who received Moderna’s Covid-19 vaccine were at higher risk of a febrile seizure within one day of vaccination than within 8-63 days after vaccination, according to a peer-reviewed study by the US Food and Drug Administration (FDA).

Among kids aged two to four, the risk was roughly two-and-a-half times higher, according to the report, published February 4 in Vaccine.

The FDA funded the study and conducted it as part of its Biologics Effectiveness and Safety (BEST) System. However, the report included a disclaimer stating that it “should not be construed to represent FDA’s views or policies.”

Karl Jablonowski, Children’s Health Defence (CHD) senior research scientist, said it was noteworthy that an FDA-led study confirming a vaccine’s downside was published in a major journal. “That likely wouldn’t have happened a few years ago,” he said.

The FDA did not respond to The Defender when asked if the agency publicly announced the febrile seizure safety signal for Moderna’s vaccine in March 2024, when the study was first published as a preprint.

Last month, the FDA asked the makers of several flu vaccines to add a warning that the vaccines can cause febrile seizures in young children.

On its website on Covid-19 vaccine safety, the Centres for Disease Control and Prevention (CDC) acknowledges that febrile seizures have been reported after vaccination with Moderna’s shot, including during Moderna’s clinical trial.

At the time of the study, both Moderna and Pfizer’s shots were authorised for emergency use for children under five. Now, Pfizer’s shot is licensed only for kids five and up. Moderna’s shot is licensed for infants and children six months and up.

However, the CDC no longer recommends routine Covid-19 vaccination for children. On October 6, 2025, the agency updated its recommendation for Covid-19 vaccination for children six months and older, advising “individual-based decision-making.”

Daniel O’Connor, founder and CEO of TrialSite News, which first reported on the FDA study, said now that there’s no blanket recommendation for Covid-19 vaccination for children, the data revealed by the FDA “helps parents weigh real benefits against real risks” of vaccination.

TrialSite News claimed the FDA’s study had “no signal” that “suggested persistent or delayed neurological harm.”

But Jablonowski called that claim “baseless” because the study didn’t evaluate long-term neurological conditions – the authors looked only at claims filed up to 63 days after vaccination.

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

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

Febrile seizures are convulsions most often caused by fevers triggered by infections related to common childhood illnesses. The seizures typically occur in children six months to five years old when they have a fever over 100.4 F.

Most febrile seizures last less than 15 minutes and are not life-threatening. According to Medpage Today, they “do not cause any permanent harm and do not have any lasting effects.”

Brian Hooker, CHD’s chief scientific officer, disagreed. “Any seizure is bad, period,” he said.

Hooker said that “‘mild’ febrile seizures can double a child’s chance of an epilepsy diagnosis and ‘complex’ febrile seizures, which last more than 15 minutes, can increase that risk up to 10 times.”

Jablonowski said a 2023 review in Frontiers in Cell and Developmental Biology showed that “febrile seizures occurring during neurodevelopment … may ‘ultimately lead to disease.’”

A 2024 review in Paediatric Neurology concluded that “febrile seizures can lead to negative lifelong consequences such as cognitive deficits, brain abnormalities, and the development of temporal lobe epilepsy in a subset of children.” The study authors analysed health claims from roughly mid-2022 to mid-2023 for kids aged two to five from three insurance companies, including Carelon Research, CVS Health and Optum. The companies accounted for roughly 17 per cent of all US paediatric claims, according to the authors.

The authors compared the number of febrile seizures reported 0-1 days after getting Moderna or Pfizer’s Covid-19 vaccine with the number reported 8-63 days after vaccination.

The Moderna vaccine showed a statistically significant increase in febrile seizures in the first day after vaccination. Pfizer’s vaccine also showed an increase, but it wasn’t statistically significant.

Neither shot showed a statistically significant increase in febrile seizures when the study authors looked at the whole first week after vaccination, rather than days 0-1. However, the authors found that the risk of both febrile and non-febrile seizures more than doubled in the week after vaccination for both the Pfizer and Moderna shots, compared to the following eight weeks – and the finding was statistically significant.

It biologically makes sense that Moderna’s shot would induce more febrile seizures than the Pfizer shot, Jablonowski said. A child’s dose of Moderna’s shot contains 25 micrograms (mcg) of mRNA while Pfizer’s shot contains only 3 mcg, he said.

“Moderna also had substantially more DNA contamination than Pfizer, which could immediately activate the innate immune system.”

• A Tell Media report / By Suzanne Burdick – Senior reporter for The Defender based in Fairfield, Iowa.