ABSTRACT
Vaccines represent one of the true medical accomplishments of human history. The discovery and implementation of vaccines against various infectious diseases has ameliorated uncountable morbidity and mortality worldwide. Introduction of any foreign substance into the body, however, carries with it the risk of rare adverse events. The fundamental goal of vaccination is to produce an antigen-specific immune response against a virulent infectious agent without actually causing the disease itself. There are several different types of vaccines, which can broadly be broken down into “live” and “non-live” vaccines. There are several hypothetical mechanisms by which vaccines could lead to neurologic adverse events, including direct neurotoxicity, molecular mimicry, immune complex formation, and loss of self-tolerance due to an aberrant immune response. An important concept in adverse events is causality—many vaccines have been temporally associated with a particular adverse event, but this is quite different than proving that a vaccine “caused” the adverse event. The 1976 formulation of the H1N1 influenza vaccine was associated with a small but statistically significant risk of developing Guillain–Barre syndrome (GBS) to the amount of one excess case of GBS per 100,000 vaccinees. Subsequent formulations of the seasonal influenza vaccine have not demonstrated a similar increase in risk of GBS. There is currently no strong evidence linking vaccines (including hepatitis B vaccine) to multiple sclerosis, or MS relapse. Acute disseminated encephalomyelitis (ADEM) is a central nervous system demyelinating disease primarily of children, which has occasionally been associated with antecedents receipt of vaccine. Initial claims of an association between measles-mumps-rubella (MMR) vaccine with autism were later discounted due to fraudulent data; unfortunately, however, the impact of this report has been ongoing concerns about a link, leading to decreased vaccination in some populations. The fact that vaccines are administered to otherwise healthy people demands that proper surveillance mechanisms for adverse events be present, and various countries employ methods for conducting pharmacovigilance. However, numerous studies conducted over decades support the safety of vaccines, with neurologic AEFI being the exception to the rule.
