title: Risk of febrile seizures after first dose of measles-mumps-rubella-varicella vaccine: a population-based cohort study.
abstract: BACKGROUND
  The combination measles-mumps-rubella-varicella (MMRV) vaccine currently used in Canada (Priorix-Tetra) may increase the risk of febrile seizures relative to the separate vaccines (MMR and varicella) previously administered. We determined the risk of febrile seizure after the first dose of MMRV, as well as any additional risk for children at high risk for seizures because of pre-existing medical conditions.
  
  
  METHODS
  In this retrospective, population-based cohort study, we compared the risk of seizures after the first dose of MMRV with the risk after same-day administration of separate MMR and varicella vaccines (MMR+V) in children 12 to 23 months of age in the province of Alberta. We deterministically linked vaccination data to health service utilization data for seizures. We used Poisson regression, with adjustment for age and calendar year, to determine the risk for the full cohort and for high-risk children.
  
  
  RESULTS
  The risk of seizures 7 to 10 days after vaccination was twice as high with MMRV as with MMR+V (relative risk [RR] 1.99, 95% confidence interval [CI] 1.30-3.05). The excess absolute risk of seizures was 3.52 seizures per 10 000 doses of MMRV relative to MMR+V. In high-risk children, the risk was not differentially higher for MMRV (RR 1.30, 95% CI 0.60-2.79).
  
  
  CONCLUSIONS
  Despite an increased risk of febrile seizures following MMRV (compared with MMR+V), the absolute level of risk was small. Policy-makers need to balance these findings with the potential benefits of administering the combination vaccine or determine whether the choice of vaccine rests with clinicians and/or parents.
authors.affiliation: Departments of Pediatrics (MacDonald) and Community Health Sciences (Simmonds, Svenson), University of Calgary, Calgary, Alta.; Faculty of Nursing (MacDonald) and School of Public Health (Svenson), University of Alberta, Edmonton, Alta.; and Epidemiology and Surveillance Team (Dover, Simmonds, Svenson), Alberta Ministry of Health, Edmonton, Alta. smacdon@ualberta.ca.
authors.forename: Shannon E
authors.initials: SE
authors.lastname: MacDonald
authors.affiliation: Departments of Pediatrics (MacDonald) and Community Health Sciences (Simmonds, Svenson), University of Calgary, Calgary, Alta.; Faculty of Nursing (MacDonald) and School of Public Health (Svenson), University of Alberta, Edmonton, Alta.; and Epidemiology and Surveillance Team (Dover, Simmonds, Svenson), Alberta Ministry of Health, Edmonton, Alta.
authors.forename: Douglas C
authors.initials: DC
authors.lastname: Dover
authors.affiliation: Departments of Pediatrics (MacDonald) and Community Health Sciences (Simmonds, Svenson), University of Calgary, Calgary, Alta.; Faculty of Nursing (MacDonald) and School of Public Health (Svenson), University of Alberta, Edmonton, Alta.; and Epidemiology and Surveillance Team (Dover, Simmonds, Svenson), Alberta Ministry of Health, Edmonton, Alta.
authors.forename: Kimberley A
authors.initials: KA
authors.lastname: Simmonds
authors.affiliation: Departments of Pediatrics (MacDonald) and Community Health Sciences (Simmonds, Svenson), University of Calgary, Calgary, Alta.; Faculty of Nursing (MacDonald) and School of Public Health (Svenson), University of Alberta, Edmonton, Alta.; and Epidemiology and Surveillance Team (Dover, Simmonds, Svenson), Alberta Ministry of Health, Edmonton, Alta.
authors.forename: Lawrence W
authors.initials: LW
authors.lastname: Svenson
mesh_terms: D000416:Alberta; D019433:Chickenpox Vaccine / Q000009:adverse effects*; D015331:Cohort Studies; D006801:Humans; D007223:Infant; D022542:Measles-Mumps-Rubella Vaccine / Q000009:adverse effects*; D016012:Poisson Distribution; D012044:Regression Analysis; D012189:Retrospective Studies; D018570:Risk Assessment; D012307:Risk Factors; D003294:Seizures, Febrile / Q000209:etiology*; D017778:Vaccines, Combined / Q000009:adverse effects
chemical_list: D019433:Chickenpox Vaccine; D022542:Measles-Mumps-Rubella Vaccine; D017778:Vaccines, Combined; C050102:measles, mumps, rubella, varicella vaccine