Two problems must be considered in regard to the relationship between vaccinations and MS: Do vaccinations favour the first attack of MS? Do they increase the short- or long-term risk in patients with known disease? Answers to these questions are difficult due to the paucity of reported cases, our ignorance of the precise frequency of neurological adverse events in
vaccines based on prospective studies, and finally by the lack of a well established pathophysiology. In most instances, the role of the
vaccine is based on a temporal link between the injection and the onset of neurological disease, and more rarely to a positive reintroduction.
Acute disseminated encephalomyelitis (ADEM), a monophasic and multifocal illness of the white and grey matter, has been observed following various viral or
bacterial infections as well as
vaccine injections for diseases such as
pertussis,
tetanus and
yellow fever. The similarities between ADEM and experimental allergic
encephalitis (EAE) are suggestive of an immunological process. In addition to the dramatic presentation of ADEM, more limited white matter involvement, such as
optic neuritis or
myelitis, has been reported following
vaccine injections, and has occasionally been counted as the first attack of MS. In France, 25 million inhabitants, almost half of the population, were vaccinated against
hepatitis B (HB) between 1991 and 1999. Several hundred cases of an acute central demyelinating event following HB vaccination were reported to the pharmacovigilance unit, leading to a modification of vaccination policy in the schools and the initiation of several studies designed to examine the possible relationship between the
vaccine and the central demyelinating events. The results of these studies failed to establish the causality of the HB
vaccine. Nevertheless, molecular mimicry between HB
antigen(s) and one or more
myelin proteins, or a non-specific activation of autoreactive lymphocytes, could constitute possible pathogenetic mechanisms for these adverse neurological events.