Seizures are temporary abnormal electro-physiologic phenomena of the brain, resulting in abnormal synchronization of electrical neuronal activity. They can manifest as an alteration in mental state, tonic or colonic movements, and various other psychic symptoms (such as déja vu or jamais vu). They are due to temporary abnormal electrical activity of a group of brain cells. The medical syndrome of recurrent, unprovoked seizures is termed epilepsy, but some seizures may occur in people who do not have epilepsy.

The treatment of epilepsy is a sub-specialty of neurology; the study of seizures is part of neuroscience, therefore the study of vaccination’s effect on the human nerve system is of particular importance.

Seizures are one the most common forms of acute vaccine reaction. Vaccination’s relationship to long-term seizures (epilepsy) is still disputed by some authorities.