Febrile seizures after COVID-19 vaccination are rare and aren’t linked to long-term health problems, contrary to implication in Gateway Pundit article

Monitoring the safety of vaccines once they are rolled out to the general population is an essential part of the vaccine development process. Doing this allows scientists to continually evaluate that a vaccine’s safety profile matches the results found in clinical trials. A wealth of epidemiological studies conducted on COVID-19 vaccines since their rollout have adhered to this practice of pharmacosurveillance.

In March 2024, researchers from the U.S. Food and Drug Administration (FDA) and private companies contributed to this effort by publishing a preprint about the effect of COVID-19 vaccines on febrile seizure, a condition that can occur in young children who develop fevers above 38°C^[1]. A preprint is a study that hasn’t yet been peer-reviewed.

Reporting on this preprint, the website The Gateway Pundit claimed it showed that “the FDA admit[ted]” that “young children [were] at a ‘significantly elevated’ risk of seizure after taking the COVID-19 vaccine”. The Gateway Pundit has repeatedly propagated disinformation about COVID-19 vaccines in the past.

Here again, the outlet’s account of the preprint leaves readers with an inaccurate understanding of the preprint’s results and conclusions. We explain why below.

What did the study do and what did it find?

The study by Forshee et al. used health insurance data to investigate whether COVID-19 mRNA vaccines increased the risk of febrile seizure in children from two to five years of age. A febrile seizure is a convulsion caused by fever. The symptoms may include disorganized movements, limb stiffening, eye rolling, and lack of responsiveness. Usually, febrile seizures don’t cause any long-term harm or health problems.

Forshee et al. set out to study this particular health condition because a previous safety study indicated that the risk of seizure could be elevated among young children following COVID-19 vaccination^[2]. However, the methods used at the time didn’t allow the researchers to draw definitive conclusions about this risk, so a more in-depth analysis was needed.

To do so, Forshee et al. collected data on children between two and four years old from health insurance databases—163,733 had received the Pfizer COVID-19 vaccine and 110,126 had received the Moderna vaccine.

The authors then compared the incidence of febrile seizure on the day of vaccination or the day after vaccination to the incidence in the period between the 8th and 63rd days after vaccination.

The reason why they chose to compare these time periods is because they hypothesized that acute conditions, like febrile seizure, are more likely to occur shortly after the event that allegedly caused them—such as vaccination—rather than many days or weeks after said event.

In other words, if the COVID-19 vaccines actually increased the risk of febrile seizure, the febrile seizure would be more likely to occur immediately after vaccination (the risk period) rather than in the following weeks (the control period).

This particular study design is called a self-controlled case series analysis and is often used in vaccine safety analysis^[3]. One advantage of this type of study is that participants act as their own control group, which eliminates the risk of bias due to differences between the test and control groups.

The authors observed 10 cases of febrile seizure during the risk period following vaccination with the Moderna vaccine and 57 during the control period. They observed seven febrile seizures during the risk period following vaccination with the Pfizer vaccine, and 81 during the control period.

The authors then calculated that the febrile seizure incidence was 2.5 times higher during the risk period than the control period among children who had received the Moderna vaccine and that this difference was statistically significant. By contrast, there was no statistically significant difference in incidence with the Pfizer vaccine.

The authors concluded their study by reaffirming that the increased risk of febrile seizure, though confirmed, didn’t modify the overall risk profile of the vaccine:

“Our study found an approximately 2.5 times increased incidence of febrile seizures among those aged 2-4 years in the 0-1 days following monovalent mRNA-1273 vaccination. However, absolute risk was low. Based on the current body of scientific evidence, the safety profile of the monovalent mRNA vaccines remains favorable for use in young children.” [emphasis added]

The study by Forshee et al. doesn’t indicate that febrile seizures following COVID-19 vaccines are a public health concern

Several aspects of The Gateway Pundit’s article on the study are misleading.

First, the article’s claim that the study found a “2.5-fold increase in febrile seizures” is based only on relative risk. Relative risk is a ratio of the incidence during the risk and control periods. In other words, it measures how high or low an incidence is relative to another one.

However, the article left out the study’s estimate of absolute risk, which is the difference between the incidence during the risk period and the control period. As we will show below, knowing the absolute risk changes a reader’s understanding of the study’s findings.

As a pedagogical document from the Boston University School of Public Health explains, relative risk “provide[s] a measure of the strength of the association between a factor and a disease or outcome”. However, it doesn’t tell us whether this is clinically important or if exposure to the factor will cause many people to develop the disease.

On the other hand, absolute risk “provides a measure of the public health impact of the risk factor, and focuses on the number of cases that could potentially be prevented by eliminating the risk factor”. Ideally, both relative risk and absolute risk are measured to assess the effectiveness and safety of a substance^[4].

To illustrate this difference between relative risk and absolute risk, let’s imagine that one in ten million people dies of a rare disease each year. That would translate to about 33 deaths per year among the 336 million people in the U.S.

Now, imagine that exposure to a particular substance doubles the incidence of that disease. That would be a 100% increase in disease incidence compared to the baseline. In other words, the relative risk is 100%.

However in absolute terms, this would only translate to 33 additional deaths per year in the U.S.. In light of the 3.4 million annual deaths that occur in the U.S., we can see that the absolute risk (33 additional deaths) is actually very small, meaning that exposure to the substance has a relatively limited impact on mortality at the population level.

Circling back to the effect of COVID-19 vaccines on febrile seizures, this means that only considering the relative risk, as The Gateway Pundit did, may exaggerate the actual danger.

Forshee et al. also calculated the absolute risk (referred to as risk difference) in the preprint:

“Though our primary meta-analysis showed a relative risk near 2.5 for febrile seizures immediately following [Moderna] vaccination, we estimated the risk difference of febrile seizures following vaccination to be small. In the 0-1 day risk interval following over 190,000 eligible [Moderna] vaccinations only 10 febrile seizure cases were observed. Our analysis found a risk difference of approximately 3 additional cases of febrile seizures per 100,000 [Moderna] vaccinations”

Put simply, febrile seizures are so rare that even the increased risk shortly after vaccination would not result in a noticeable increase in cases.

Furthermore, The Gateway Pundit failed to report that this increase in risk diminishes rapidly in the days and weeks following vaccination. Indeed, Forshee et al. only observed a relative risk increase when the risk period was defined as the day of vaccination and the day after vaccination. When the risk period was extended to the first seven days following vaccination, the statistically significant risk increase disappeared.

Finally, the authors also warned that “the range from the confidence interval for the estimate included the potential for fewer than zero febrile seizure events per 100,000 vaccinations”. This means that it is also possible for the Moderna vaccine to produce no additional cases of febrile seizure at all. This is yet another piece of information missing from The Gateway Pundit article that detracts from its claim.

Febrile seizures don’t carry long-term health risks and are less common with COVID-19 vaccines than with some other childhood vaccines

The Gateway Pundit claimed that the results from Forshee et al. “raise further concerns” about COVID-19 vaccines, implying that COVID-19 vaccines put young children at risk of a serious health condition. However, what they fail to mention is that febrile seizures are usually harmless.

Febrile seizures occur as a result of uncoordinated electrical activity in the brain. Unlike other types of seizures, which may occur due to serious health conditions such as meningitis, concussion, and brain tumors, febrile seizures typically occur during fevers above 38°C. Febrile seizures are also different from epilepsy, where people repeatedly suffer from unprovoked seizures.

According to healthcare institutions, febrile seizures are more frightening than actually dangerous.

The Mount Sinai Medical Center explains:

“A febrile seizure can be frightening for any parent or caregiver. Most of the time, a febrile seizure does not cause any harm. The child usually does not have a more serious long-term health problem.”.

The Mayo Clinic explains:

“It can be frightening when your child has a febrile seizure. Fortunately, febrile seizures are usually harmless, only last a few minutes, and typically don’t indicate a serious health problem.”.

That febrile seizures sometimes occur after vaccination isn’t unexpected. Fever is a common reaction to vaccines due to the immune system’s reaction to the vaccine. Therefore, it’s plausible that, in some cases, a vaccine-induced fever can cause a febrile seizure.

In fact, this is a known side effect associated with other vaccines, particularly childhood vaccines. Furthermore, some childhood vaccines are associated with a higher risk of febrile seizure than COVID-19 mRNA vaccines. According to the U.S. Centers for Disease Control and Prevention (CDC), any combination of influenza, pneumococcal, and DTaP vaccines is associated with a risk of up to 30 febrile seizures per 100,000 vaccinated children. Yet, this risk is still considered to be small.

Finally, febrile seizures aren’t unique to vaccines. Since certain infections, like COVID-19 can cause fever, infection can also cause febrile seizure^[5-10].

In summary, febrile seizures are usually harmless and aren’t associated with any long-term serious repercussions. However, by failing to provide this information, The Gateway Pundit’s article gives readers a different impression, implying incorrectly that febrile seizures caused by COVID-19 vaccines pose a health threat.

REFERENCES

• 1 – Forshee et al. (2024) Evaluation of Febrile Seizure Risk Following Ancestral Monovalent COVID-19 mRNA Vaccination Among U.S. Children Aged 2-5 Years. MedRxiv (preprint)
• 2 – Hu et al. (2023) Safety of Monovalent BNT162b2 (Pfizer-BioNTech), mRNA-1273 (Moderna), and NVX-CoV2373 (Novavax) COVID-19 Vaccines in US Children Aged 6 months to 17 years. MedRxiv (preprint).
• 3 – Petersen et al. (2016) Self controlled case series methods: an alternative to standard epidemiological study designs. The British Medical Journal.
• 4 – Noordzij et al. (2017) Relative risk versus absolute risk: one cannot be interpreted without the other. Nephrology Dialysis Transplantation.
• 5 – Xu et al. (2023) Febrile seizure in children with COVID-19 during the Omicron wave. Frontiers in pediatrics.
• 6 – Cadet et al. (2022) Evaluation of Febrile Seizure Diagnoses Associated With COVID-19. Journal of Child Neurology.
• 7 – Hanlon et al. (2023) The Association Between COVID-19 and Febrile Seizure. Pediatric Emergency Care.
• 8 – Hautala et al. (2021) Respiratory viruses and febrile response in children with febrile seizures: A cohort study and embedded case-control study. Seizure.
• 9 – Francis et al. (2016) An observational study of febrile seizures: the importance of viral infection and immunization. BMC Pediatrics.
• 10 – Han & Han (2020) Febrile Seizures and Respiratory Viruses Determined by Multiplex Polymerase Chain Reaction Test and Clinical Diagnosis. Children.