title: Transmission dynamics and risk factors for pandemic H1N1-related illness: outbreak investigation in a rural community of British Columbia, Canada.
abstract: OBJECTIVE
  To characterize the first-wave epidemiologic features of influenza-like illness (ILI) associated with the novel pandemic A/H1N1 [A(H1N1)pdm09] virus.
  
  
  METHODS
  We used generalized linear mixed models (GLMM) to assess risk factors and non-parametric and/or parametric distributions to estimate attack rates, secondary attack rates (SAR), duration of illness, and serial interval during a laboratory-confirmed community outbreak of A(H1N1)pdm09 clustered around on-reserve residents and households of an elementary school in rural British Columbia, Canada, in late April/early May 2009. ILI details were collected as part of outbreak investigation by community telephone survey in early June 2009.
  
  
  RESULTS
  Overall, 92/408 (23%) of participants developed ILI and 36/408 (9%) experienced medically attended ILI (MAILI). The overall SAR in households was 22%: highest among participants 1-4 years of age (yoa) (50%) followed by < 1 yoa (38%), 5-8 yoa (20%), 10-19 yoa (13%), 20-49 yoa (20%), and 50-64 yoa (0%). The median serial interval was estimated at 3·5 days (95% CI: 2·1-5·1). In multivariable GLMM analysis, having a chronic condition (OR: 2·58; 95% CI: 1·1-6·04), younger age [1-8 yoa: OR: 4·63; 95% CI: 2·25-9·52; 9-19 yoa: OR: 1·95; 95% CI: 0·97-3·9 (referent: ≥ 20 yoa)] and receipt of 2008-2009 influenza vaccine (OR: 2·68; 95% CI: 1·37-5·25) were associated with increased risk of ILI. Median duration of illness was 9 days, longer among those with chronic conditions (21 days). Median time to seeking care after developing illness was 4·5 days. On-reserve participants had higher chronic conditions, household density, ILI, MAILI, and SAR.
  
  
  CONCLUSIONS
  During a community outbreak of A(H1N1)pdm09-related illness, we identified substantial clinical ILI attack rates exceeding 20% with secondary household attack rates as high as 50% in young children. The serial interval was short suggesting a narrow period to prevent transmission.
authors.affiliation: BC Centre for Disease Control, Vancouver, BC, Canada. naveed.janjua@bccdc.ca
authors.forename: Naveed Z
authors.initials: NZ
authors.lastname: Janjua
authors.forename: Danuta M
authors.initials: DM
authors.lastname: Skowronski
authors.forename: Travis S
authors.initials: TS
authors.lastname: Hottes
authors.forename: William
authors.initials: W
authors.lastname: Osei
authors.forename: Evan
authors.initials: E
authors.lastname: Adams
authors.forename: Martin
authors.initials: M
authors.lastname: Petric
authors.forename: Marcus
authors.initials: M
authors.lastname: Lem
authors.forename: Patrick
authors.initials: P
authors.lastname: Tang
authors.forename: Gaston
authors.initials: G
authors.lastname: De Serres
authors.forename: David M
authors.initials: DM
authors.lastname: Patrick
authors.forename: David
authors.initials: D
authors.lastname: Bowering
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001955:British Columbia / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D004196:Disease Outbreaks; D005191:Family Characteristics; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission* / Q000821:virology; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D012307:Risk Factors; D012424:Rural Population; D012574:Schools; D055815:Young Adult