A seizure is defined as a sudden, uncontrolled electrical disturbance in the brain that temporarily alters mental status, movements, or consciousness. The introduction of the COVID-19 vaccines led to widespread public discussion regarding their safety, particularly concerning potential neurological side effects like seizures. Understanding the actual incidence rates and the biological context of these events is important for a clear assessment of vaccine safety.
Safety Surveillance Data on Incidence Rates
Large-scale safety monitoring systems have investigated the incidence of seizures following COVID-19 vaccination. Data from passive surveillance systems like the Vaccine Adverse Event Reporting System (VAERS) show new-onset seizures at a very low estimated rate, approximately 3.19 per 100,000 persons per year. Population-based studies, such as those conducted in Hong Kong, generally found no increased risk of seizure within the immediate week following administration of both mRNA and inactivated vaccines.
A small signal for seizures was detected in very young children (aged two to five years) following certain mRNA vaccines. However, the vast majority of these reports were classified as febrile seizures, which are temporary events triggered by fever and not considered true epilepsy. When comparing the rates of new-onset seizures after COVID-19 vaccines to other common vaccines, the risk signal often disappears, indicating these events are rare and consistent with other immunizations.
Distinguishing Seizures from Syncope
A challenge in interpreting adverse event reports is the frequent confusion between a true epileptic seizure and syncope, or fainting. Syncope is a common, temporary loss of consciousness caused by a sudden, brief reduction in blood flow to the brain. This is often triggered by anxiety, fear of needles, or pain related to the injection itself, known as a vasovagal response.
During a syncopal episode, the temporary lack of oxygen to the brain can sometimes cause brief, involuntary muscle jerks or twitching, called convulsive syncope. These movements can be easily mistaken for a seizure by observers, leading to over-reporting in surveillance systems. Unlike a true seizure, recovery from syncope is typically rapid and complete within moments, and the event is related to the injection process, not a biological reaction to the vaccine ingredients.
Biological Plausibility of Vaccine-Related Neurological Events
The question of how a vaccine could theoretically trigger a neurological event centers on the body’s immune response. Vaccines are designed to induce systemic inflammation to train the immune system to recognize the virus. This temporary, controlled inflammation can, in rare instances, lead to neurological symptoms.
One well-established mechanism is the link between fever and febrile seizures, which are the most common type reported in young children after vaccination. Fever is a common side effect of many vaccines, and in children predisposed to them, this temperature spike can trigger a short, self-limiting seizure. Other, much rarer theoretical mechanisms involve immune cross-reactivity, where the immune response mistakenly targets similar proteins in the nervous system, known as molecular mimicry. However, the vast majority of neurological symptoms reported are mild and transient, reflecting a normal immune system activation.
Comparing Seizure Risk: Vaccine vs. COVID-19 Infection
Contextualizing the small risk associated with vaccination requires comparing it to the risks posed by the COVID-19 infection itself. Contracting the SARS-CoV-2 virus is associated with a significantly higher risk of various neurological complications, including new-onset seizures and strokes. The infection can trigger seizures indirectly through mechanisms like increased levels of pro-inflammatory molecules, which disrupt normal brain function.
Data from multiple studies show that the risk of neurological complications after COVID-19 infection is estimated to be several hundred times higher than any potential risk associated with the vaccine. For individuals with pre-existing epilepsy, the frequency of seizures increased significantly more often following the actual COVID-19 infection than after receiving the vaccine. Vaccinating is a means of reducing overall neurological risk.