Ataxia: An inability to coordinate muscle activity during voluntary movement; most often results from disorders of the cerebellum or the posterior columns of the spinal cord; may involve the limbs, head, or trunk.1

Autism: A developmental disorder that appears in the first 3 years of life, and affects the brain’s normal development of social and communication skills.2

Autoimmune hepatitis: Chronic liver disease of unknown etiology characterized by histologic presence of periportal hepatitis with plasma cell infiltration, hypergammaglobulinemia, and presence of autoantibodies in the serum.1

Bell’s palsy: Paresis or paralysis, usually unilateral, of the facial muscles, caused by dysfunction of the seventh cranial nerve; probably due to a viral infection; usually demyelinating in type.1

Bias: Systematic deviation of results or inferences from truth; processes leading to such deviation. An error in the conception and design of a study—or in the collection, analysis, interpretation, reporting, publication, or review of data—leading to results or conclusions that are systematically (as opposed to randomly) different from the truth.4

Brachial neuritis: Inflammation of nerves in the arm causing muscle weakness and pain.5

Case-crossover study: The method is akin to a matched case-control study, in which the controls are sampled from the person-time of the case prior to the event. Vaccinations are ascertained in a defined time period immediately prior to the event and in one or several earlier control periods of the same duration. This produces, for each case, a matched set of exposure variables corresponding to the event and control periods, which may be analyzed as in a case-control study.6

Case-control study: The observational epidemiologic study of persons with the disease (or another outcome variable) of interest and a suitable control group of persons without the disease (comparison group, reference group). The potential relationship of a suspected risk factor or an attribute to the disease is examined by comparing the diseased and nondiseased subjects with regard to how frequently the factor or attribute is present (or, if quantitative, the levels of the attribute) in each of the groups (diseased and nondiseased).4

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