title: Association between birth order and emergency room visits and acute hospital admissions following pediatric vaccination: a self-controlled study.
abstract: OBJECTIVE
  We investigated the association between a child's birth order and emergency room (ER) visits and hospital admissions following 2-,4-,6- and 12-month pediatric vaccinations.
  
  
  METHODS
  We included all children born in Ontario between April 1(st), 2006 and March 31(st), 2009 who received a qualifying vaccination. We identified vaccinations, ER visits and admissions using health administrative data housed at the Institute for Clinical Evaluative Sciences. We used the self-controlled case series design to compare the relative incidence (RI) of events among 1(st)-born and later-born children using relative incidence ratios (RIR).
  
  
  RESULTS
  For the 2-month vaccination, the RIR for 1(st)-borns versus later-born children was 1.37 (95% CI: 1.19-1.57), which translates to 112 additional events/100,000 vaccinated. For the 4-month vaccination, the RIR for 1(st)-borns vs. later-borns was 1.70 (95% CI: 1.45-1.99), representing 157 additional events/100,000 vaccinated. At 6 months, the RIR for 1(st) vs. later-borns was 1.27 (95% CI: 1.09-1.48), or 77 excess events/100,000 vaccinated. At the 12-month vaccination, the RIR was 1.11 (95% CI: 1.02-1.21), or 249 excess events/100,000 vaccinated.
  
  
  CONCLUSIONS
  Birth order is associated with increased incidence of ER visits and hospitalizations following vaccination in infancy. 1(st)-born children had significantly higher relative incidence of events compared to later-born children.
authors: ICES uOttawa, Ottawa, Ontario, Canada ; Department of Clinical Epidemiology, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada ; Department of Epidemiology and Community Medicine, University of Ottawa, Ottawa, Ontario, Canada.
authors: Steven
authors: S
authors: Hawken
authors: Jeffrey C
authors: JC
authors: Kwong
authors: Shelley L
authors: SL
authors: Deeks
authors: Natasha S
authors: NS
authors: Crowcroft
authors: Robin
authors: R
authors: Ducharme
authors: Douglas G
authors: DG
authors: Manuel
authors: Kumanan
authors: K
authors: Wilson
mesh_terms: D001722:Birth Order; D002648:Child; D002675:Child, Preschool; D015721:Diphtheria-Tetanus-Pertussis Vaccine / Q000009:adverse effects*; D004636:Emergency Service, Hospital / Q000706:statistics & numerical data*; D005260:Female; D018073:Haemophilus Vaccines / Q000009:adverse effects*; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D015994:Incidence; D007223:Infant; D008297:Male; D009864:Ontario / Q000453:epidemiology; D011054:Poliovirus Vaccine, Inactivated / Q000009:adverse effects*; D014611:Vaccination; D018074:Vaccines, Conjugate / Q000009:adverse effects
chemical_list: D015721:Diphtheria-Tetanus-Pertussis Vaccine; D018073:Haemophilus Vaccines; D011054:Poliovirus Vaccine, Inactivated; D018074:Vaccines, Conjugate; C541234:diphtheria-tetanus-five component acellular pertussis-inactivated poliomyelitis -Haemophilus influenzae type b conjugate vaccine