title: The validity of self-initiated, event-driven infectious disease reporting in general population cohorts.
abstract: BACKGROUND
  The 2009/2010 pandemic influenza highlighted the need for valid and timely incidence data. In 2007 we started the development of a passive surveillance scheme based on passive follow-up of representative general population cohorts. Cohort members are asked to spontaneously report all instances of colds and fevers as soon as they occur for up to 9 months. Suspecting that compliance might be poor, we aimed to assess the validity of self-initiated, event-driven outcome reporting over long periods.
  
  
  METHODS
  During two 8 week periods in 2008 and 2009, 2376 and 2514 cohort members in Stockholm County were sent one-week recall questionnaires, which served as reference method.
  
  
  RESULTS
  The questionnaires were completed by 88% and 86% of the cohort members. Whilst the false positive proportion (1-specificity) in the reporting was low (upper bound of the 95% confidence interval [CI] ≤ 2% in each season), the false negative proportion (failure to report, 1-sensitivity) was considerable (60% [95% CI 52%-67%] in each season). Still, the resulting epidemic curves for influenza-like illness compared well with those from existing General Practitioner-based sentinel surveillance in terms of shape, timing of peak, and year-to-year variation. This suggested that the error was fairly constant.
  
  
  CONCLUSIONS
  Passive long-term surveillance through self-initiated, event-driven outcome reporting underestimates incidence rates of common upper respiratory tract infections. However, because underreporting appears predictable, simple corrections could potentially restore validity.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Hanna
authors.initials: H
authors.lastname: Merk
authors.forename: Sharon
authors.initials: S
authors.lastname: Kühlmann-Berenzon
authors.forename: Christin
authors.initials: C
authors.lastname: Bexelius
authors.forename: Sven
authors.initials: S
authors.lastname: Sandin
authors.forename: Jan-Eric
authors.initials: JE
authors.lastname: Litton
authors.forename: Annika
authors.initials: A
authors.lastname: Linde
authors.forename: Olof
authors.initials: O
authors.lastname: Nyrén
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015982:Bias; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D018563:Disease Notification / Q000706:statistics & numerical data*; D005189:False Positive Reactions; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007251:Influenza, Human / Q000453:epidemiology*; D016015:Logistic Models; D008875:Middle Aged; D015999:Multivariate Analysis; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D012621:Seasons; D018571:Sentinel Surveillance; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult