Immunization Safety Review Influenza Vaccines and Neurological Complications (2004) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-22
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The Immunization Safety Review committee reviewed the data on influenza vaccine and neurological conditions and concluded that the evidence favored acceptance of a causal relationship between the 1976 swine influenza vaccine and GBS in adults. The evidence about GBS for other years' influenza vaccines is not clear one way or the other (that is, the evidence is inadequate to accept or reject a causal relationships.
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The committee reviews three immunization safety review topics each year, addressing one at a time. In this seventh report in the series, the committee examines the hypothesis that influenza vaccines are associated with an increased risk of neurological complications, particularly Guillain-Barre syndrome (GBS)
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A list of the materials reviewed by the committee, including many items not cited in this report, can be found on the project's website. THE FRAMEWORK FOR SCIENTIFIC ASSESSMENT Causality The Immunization Safety Review Committee has adopted the framework for assessing causality developed by previous IOM committees (IOM, 1991; 1994a,b)
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of the evidence. Influenza Vaccines and Neurological Complications The committee's review of the evidence concerning risks that might be associated with influenza vaccines had to take into account a distinctive feature of the vaccine: its formulation changes from year to year to reflect changes in the strains of influenza virus circulating in the population.
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Vaccination is the primary means of reducing the impact of influenza. The effectiveness of influenza vaccines depends, in part, on the match between the viral strains used to produce them and the strains that actually circulate in the subsequent influenza season.
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Exposure to certain vaccines has also been associated with an increased risk for GBS. The potential association between GBS and influenza vaccines, most notably the 1976 swine influenza vaccine, has been widely studied and is the subject of this report.
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or MS. SCIENTIFIC ASSESSMENT Causality Guillain-Barre' Syndrome For its review of the epidemiologic evidence regarding a possible association between influenza vaccination and GBS, the committee separated studies concern
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The committee concludes that the evidence favors acceptance of a causal relationship between 1976 swine influenza vaccine and Guillain-Barre syndrome in adults. Concerns that the evidence of increased risk found in the original analysis of the national data might have been a reflection of inaccuracies in ascertainment of GBS cases have been addressed in subsequent studies by detailed and systematic reviews of clinical data to verify GBS diagnoses.
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It found no increase in the risk of onset of MS associated with influenza vaccination, but in the absence of confirmation from other sources, the committee concludes that the evidence is inadequate to accept or reject a causal relationship between influenza vaccines and incident MS in adults. However, the biological mechanisms involved in the onset of MS are presumed to be related to those involved in relapse.
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No other epidemiological studies were identified. Based on the nature of case reports and the paucity of epidemiological data, the committee concludes that the evidence is inadequate to accept or reject a causal relationship between influenza vaccines and other demyelinating neurological disorders.
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Thus, based on the lack of direct published evidence on influenza vaccines and demyelinating neurological disorders in children, especially those aged 6-23 months, the committee concludes that there is no evidence bearing on a causal relationship between influenza vaccines and demyelinating neurological disorders in children aged 6-23 months. Biological Mechanisms In its assessment of the possibility of a relationship between influenza vaccines and neurological complications, the committee hypothesized two general ways vaccine could lead to neurological complications: immune-mediated processes and neurotoxic effects.
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As discussed in the subsequent section, however, the evidence in support of this theory is limited, and some is indirect. The following biological evidence relates to the theory that influenza vaccines could be associated with neurological complications: · Bystander activation.
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The committee concludes that there is weak evidence for biological mechanisms related to immune-mediated processes, including molecular mimicry and bystander activation, by which receipt of any influenza vaccine could possibly influence an individual's risk of developing the neurological complications of GBS, MS, or other demyelinating conditions such as optic neuritis. In the absence of experimental or human evidence regarding the direct neurotoxic effect of influenza vaccines, the committee concludes that this mechanism is only theoretical.
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Research With a vaccine as widely used as influenza vaccine, the committee considers it important to pursue research and research-related activities aimed at ensuring that any risk of GBS or other neurological complications is minimized. Surveillance and Epidemiological Studies Even though use of the vaccine generally appears to pose minimal risk of adverse neurological events, the strong association between the 1976 vaccine and GBS points to the need for appropriate vigilance through adequate surveillance systems and for better tools to support studies of rare adverse events.
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To gain further insight into these mechanisms, the committee sees a need for additional basic and clinical research on influenza viruses, the composition and immunological properties of the 1976 vaccine, immunological responses to vaccines in general, and host characteristics that may affect susceptibility to adverse events. There is a need to better understand the immunological responses in recipients of the 1976 swine influenza vaccine who experienced GBS.
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have provided at least some basis for considering bystander activation as a potential mechanism by which influenza vaccines could cause GBS or related neurological complications. As it did in a previous report (IOM, 2002a)
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1978. Swine influenza vaccination in patients with multiple sclerosis.
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1984. An epidemiologic and clinical evaluation of Guillain-Barre syndrome reported in association with the administration of swine influenza vaccines.
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1991. Reassessment of the association between Guillain-Barre syndrome and receipt of swine influenza vaccine in 1976-1977: results of a two-state study.
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THE FRAMEWORK FOR SCIENTIFIC ASSESSMENT Causality The Immunization Safety Review Committee has adopted the framework for assessing causality cleveloped by previous TOM committees (IOM, 1991; 1994a,b) convened uncler the congressional mandate of P.~.
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