title: Probabilistic daily ILI syndromic surveillance with a spatio-temporal Bayesian hierarchical model.
abstract: BACKGROUND
  For daily syndromic surveillance to be effective, an efficient and sensible algorithm would be expected to detect aberrations in influenza illness, and alert public health workers prior to any impending epidemic. This detection or alert surely contains uncertainty, and thus should be evaluated with a proper probabilistic measure. However, traditional monitoring mechanisms simply provide a binary alert, failing to adequately address this uncertainty.
  
  
  RESULTS
  Based on the Bayesian posterior probability of influenza-like illness (ILI) visits, the intensity of outbreak can be directly assessed. The numbers of daily emergency room ILI visits at five community hospitals in Taipei City during 2006-2007 were collected and fitted with a Bayesian hierarchical model containing meteorological factors such as temperature and vapor pressure, spatial interaction with conditional autoregressive structure, weekend and holiday effects, seasonality factors, and previous ILI visits. The proposed algorithm recommends an alert for action if the posterior probability is larger than 70%. External data from January to February of 2008 were retained for validation. The decision rule detects successfully the peak in the validation period. When comparing the posterior probability evaluation with the modified Cusum method, results show that the proposed method is able to detect the signals 1-2 days prior to the rise of ILI visits.
  
  
  CONCLUSIONS
  This Bayesian hierarchical model not only constitutes a dynamic surveillance system but also constructs a stochastic evaluation of the need to call for alert. The monitoring mechanism provides earlier detection as well as a complementary tool for current surveillance programs.
authors.affiliation: Institute of Epidemiology, College of Public Health, National Taiwan University, Taipei, Taiwan.
authors.forename: Ta-Chien
authors.initials: TC
authors.lastname: Chan
authors.forename: Chwan-Chuen
authors.initials: CC
authors.lastname: King
authors.forename: Muh-Yong
authors.initials: MY
authors.lastname: Yen
authors.forename: Po-Huang
authors.initials: PH
authors.lastname: Chiang
authors.forename: Chao-Sheng
authors.initials: CS
authors.lastname: Huang
authors.forename: Chuhsing K
authors.initials: CK
authors.lastname: Hsiao
mesh_terms: D000465:Algorithms; D001499:Bayes Theorem; D006801:Humans; D007251:Influenza, Human / Q000453:epidemiology*