title: Assessing secondary attack rates among household contacts at the beginning of the influenza A (H1N1) pandemic in Ontario, Canada, April-June 2009: a prospective, observational study.
abstract: BACKGROUND
  Understanding transmission dynamics of the pandemic influenza A (H1N1) virus in various exposure settings and determining whether transmissibility differed from seasonal influenza viruses was a priority for decision making on mitigation strategies at the beginning of the pandemic. The objective of this study was to estimate household secondary attack rates for pandemic influenza in a susceptible population where control measures had yet to be implemented.
  
  
  METHODS
  All Ontario local health units were invited to participate; seven health units volunteered. For all laboratory-confirmed cases reported between April 24 and June 18, 2009, participating health units performed contact tracing to detect secondary cases among household contacts. In total, 87 cases and 266 household contacts were included in this study. Secondary cases were defined as any household member with new onset of acute respiratory illness (fever or two or more respiratory symptoms) or influenza-like illness (fever plus one additional respiratory symptom). Attack rates were estimated using both case definitions.
  
  
  RESULTS
  Secondary attack rates were estimated at 10.3% (95% CI 6.8-14.7) for secondary cases with influenza-like illness and 20.2% (95% CI 15.4-25.6) for secondary cases with acute respiratory illness. For both case definitions, attack rates were significantly higher in children under 16 years than adults (25.4% and 42.4% compared to 7.6% and 17.2%). The median time between symptom onset in the primary case and the secondary case was estimated at 3.0 days.
  
  
  CONCLUSIONS
  Secondary attack rates for pandemic influenza A (H1N1) were comparable to seasonal influenza estimates suggesting similarities in transmission. High secondary attack rates in children provide additional support for increased susceptibility to infection.
authors.affiliation: Ontario Agency for Health Protection and Promotion, Toronto, Ontario, Canada. rachel.savage@oahpp.ca
authors.forename: Rachel
authors.initials: R
authors.lastname: Savage
authors.forename: Michael
authors.initials: M
authors.lastname: Whelan
authors.forename: Ian
authors.initials: I
authors.lastname: Johnson
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Rea
authors.forename: Marie
authors.initials: M
authors.lastname: LaFreniere
authors.forename: Laura C
authors.initials: LC
authors.lastname: Rosella
authors.forename: Freda
authors.initials: F
authors.lastname: Lam
authors.forename: Tina
authors.initials: T
authors.lastname: Badiani
authors.forename: Anne-Luise
authors.initials: AL
authors.lastname: Winter
authors.forename: Deborah J
authors.initials: DJ
authors.lastname: Carr
authors.forename: Crystal
authors.initials: C
authors.lastname: Frenette
authors.forename: Maureen
authors.initials: M
authors.lastname: Horn
authors.forename: Kathleen
authors.initials: K
authors.lastname: Dooling
authors.forename: Monali
authors.initials: M
authors.lastname: Varia
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Holt
authors.forename: Vidya
authors.initials: V
authors.lastname: Sunil
authors.forename: Catherine
authors.initials: C
authors.lastname: Grift
authors.forename: Eleanor
authors.initials: E
authors.lastname: Paget
authors.forename: Michael
authors.initials: M
authors.lastname: King
authors.forename: John
authors.initials: J
authors.lastname: Barbaro
authors.forename: Natasha S
authors.initials: NS
authors.lastname: Crowcroft
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D016358:Contact Tracing; D005191:Family Characteristics; D005260:Female; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission; D008297:Male; D008875:Middle Aged; D009864:Ontario / Q000453:epidemiology; D058873:Pandemics; D011159:Population Surveillance; D011446:Prospective Studies; D012307:Risk Factors; D012621:Seasons; D055815:Young Adult