title: Causality assessment of serious neurologic adverse events following 2009 H1N1 vaccination.
abstract: BACKGROUND
  Adverse events occurring after vaccination are routinely reported to the Vaccine Adverse Event Reporting System (VAERS). We studied serious adverse events (SAEs) of a neurologic nature reported after receipt of influenza A (H1N1) 2009 monovalent vaccine during the 2009-2010 influenza season. Investigators in the Clinical Immunization Safety Assessment (CISA) network sought to characterize these SAEs and to assess their possible causal relationship to vaccination.
  
  
  METHODS
  Centers for Disease Control and Prevention (CDC) and Food and Drug Administration (FDA) physicians reviewed all SAE reports (as defined by the Code of Federal Regulations, 21CFR§314.80) after receipt of H1N1 vaccine reported to VAERS between October 1, 2009 and March 31, 2010. Non-fatal SAE reports with neurologic presentation were referred to CISA investigators, who requested and reviewed additional medical records and clinical information as available. CISA investigators assessed the causal relationship between vaccination and the event using modified WHO criteria as defined.
  
  
  RESULTS
  212 VAERS reports of non-fatal serious neurological events were referred for CISA review. Case reports were equally distributed by gender (50.9% female) with an age range of 6 months to 83 years (median 38 years). The most frequent diagnoses reviewed were: Guillain-Barré Syndrome (37.3%), seizures (10.8%), cranial neuropathy (5.7%), and acute disseminated encephalomyelitis (3.8%). Causality assessment resulted in classification of 72 events as "possibly" related (33%), 108 as "unlikely" related (51%), and 20 as "unrelated" (9%) to H1N1 vaccination; none were classified as "probable" or "definite" and 12 were unclassifiable (6%).
  
  
  CONCLUSIONS
  The absence of a specific test to indicate whether a vaccine component contributes to the pathogenesis of an event occurring within a biologically plausible time period makes assessing causality difficult. The development of standardized protocols for providers to use in evaluation of adverse events following immunization, and rapid identification and follow-up of VAERS reports could improve causality assessment.
authors: Vanderbilt Vaccine Research Program, Vanderbilt University Medical Center, Nashville, TN, USA. elizabeth.williams@vanderbilt.edu
authors: S Elizabeth
authors: SE
authors: Williams
authors: Barbara A
authors: BA
authors: Pahud
authors: Claudia
authors: C
authors: Vellozzi
authors: Peter D
authors: PD
authors: Donofrio
authors: Cornelia L
authors: CL
authors: Dekker
authors: Neal
authors: N
authors: Halsey
authors: Nicola P
authors: NP
authors: Klein
authors: Roger P
authors: RP
authors: Baxter
authors: Colin D
authors: CD
authors: Marchant
authors: Philip S
authors: PS
authors: Larussa
authors: Elizabeth D
authors: ED
authors: Barnett
authors: Jerome I
authors: JI
authors: Tokars
authors: Brian E
authors: BE
authors: McGeeney
authors: Robert C
authors: RC
authors: Sparks
authors: Laurie L
authors: LL
authors: Aukes
authors: Kathleen
authors: K
authors: Jakob
authors: Silvia
authors: S
authors: Coronel
authors: James J
authors: JJ
authors: Sejvar
authors: Barbara A
authors: BA
authors: Slade
authors: Kathryn M
authors: KM
authors: Edwards
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D003389:Cranial Nerve Diseases / Q000139:chemically induced / Q000453:epidemiology; D004673:Encephalomyelitis, Acute Disseminated / Q000139:chemically induced / Q000453:epidemiology; D005260:Female; D020275:Guillain-Barre Syndrome / Q000139:chemically induced / Q000453:epidemiology; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007252:Influenza Vaccines / Q000008:administration & dosage / Q000009:adverse effects*; D008297:Male; D008875:Middle Aged; D009422:Nervous System Diseases / Q000139:chemically induced* / Q000453:epidemiology*; D014611:Vaccination / Q000009:adverse effects*; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines