title: Adverse events following 12 and 18 month vaccinations: a population-based, self-controlled case series analysis.
abstract: BACKGROUND
  Live vaccines have distinct safety profiles, potentially causing systemic reactions one to 2 weeks after administration. In the province of Ontario, Canada, live MMR vaccine is currently recommended at age 12 months and 18 months.
  
  
  METHODS
  Using the self-controlled case series design we examined 271,495 12 month vaccinations and 184,312 18 month vaccinations to examine the relative incidence of the composite endpoint of emergency room visits or hospital admissions in consecutive one day intervals following vaccination. These were compared to a control period 20 to 28 days later. In a post-hoc analysis we examined the reasons for emergency room visits and the average acuity score at presentation for children during the at-risk period following the 12 month vaccine.
  
  
  RESULTS
  Four to 12 days post 12 month vaccination, children had a 1.33 (1.29-1.38) increased relative incidence of the combined endpoint compared to the control period, or at least one event during the risk interval for every 168 children vaccinated. Ten to 12 days post 18 month vaccination, the relative incidence was 1.25 (95%, 1.17-1.33) which represented at least one excess event for every 730 children vaccinated. The primary reason for increased events was statistically significant elevations in emergency room visits following all vaccinations. There were non-significant increases in hospital admissions. There were an additional 20 febrile seizures for every 100,000 vaccinated at 12 months.
  
  
  CONCLUSIONS
  There are significantly elevated risks of primarily emergency room visits approximately one to two weeks following 12 and 18 month vaccination. Future studies should examine whether these events could be predicted or prevented.
authors.affiliation: Department of Medicine, Ottawa Hospital Research Institute, University of Ottawa, Ottawa, Canada. kwilson@ohri.ca
authors.forename: Kumanan
authors.initials: K
authors.lastname: Wilson
authors.forename: Steven
authors.initials: S
authors.lastname: Hawken
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Kwong
authors.forename: Shelley
authors.initials: S
authors.lastname: Deeks
authors.forename: Natasha S
authors.initials: NS
authors.lastname: Crowcroft
authors.forename: Carl
authors.initials: C
authors.lastname: Van Walraven
authors.forename: Beth K
authors.initials: BK
authors.lastname: Potter
authors.forename: Pranesh
authors.initials: P
authors.lastname: Chakraborty
authors.forename: Jennifer
authors.initials: J
authors.lastname: Keelan
authors.forename: Michael
authors.initials: M
authors.lastname: Pluscauskas
authors.forename: Doug
authors.initials: D
authors.lastname: Manuel
mesh_terms: D023381:Endpoint Determination; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007223:Infant; D011159:Population Surveillance; D012307:Risk Factors; D013997:Time Factors; D014611:Vaccination / Q000009:adverse effects*