title: Modeling the cost of influenza: the impact of missing costs of unreported complications and sick leave.
abstract: BACKGROUND
  Estimating the economic impact of influenza is complicated because the disease may have non-specific symptoms, and many patients with influenza are registered with other diagnoses. Furthermore, in some countries like Norway, employees can be on paid sick leave for a specified number of days without a doctor's certificate ("self-reported sick leave") and these sick leaves are not registered. Both problems result in gaps in the existing literature: costs associated with influenza-related illness and self-reported sick leave are rarely included. The aim of this study was to improve estimates of total influenza-related health-care costs and productivity losses by estimating these missing costs.
  
  
  METHODS
  Using Norwegian data, the weekly numbers of influenza-attributable hospital admissions and certified sick leaves registered with other diagnoses were estimated from influenza-like illness surveillance data using quasi-Poisson regression. The number of self-reported sick leaves was estimated using a Monte-Carlo simulation model of illness recovery curves based on the number of certified sick leaves. A probabilistic sensitivity analysis was conducted on the economic outcomes.
  
  
  RESULTS
  During the 1998/99 through 2005/06 influenza seasons, the models estimated an annual average of 2700 excess influenza-associated hospitalizations in Norway, of which 16% were registered as influenza, 51% as pneumonia and 33% were registered with other diagnoses. The direct cost of seasonal influenza totaled US$22 million annually, including costs of pharmaceuticals and outpatient services. The annual average number of working days lost was predicted at 793 000, resulting in an estimated productivity loss of US$231 million. Self-reported sick leave accounted for approximately one-third of the total indirect cost. During a pandemic, the total cost could rise to over US$800 million.
  
  
  CONCLUSIONS
  Influenza places a considerable burden on patients and society with indirect costs greatly exceeding direct costs. The cost of influenza-attributable complications and the cost of self-reported sick leave represent a considerable part of the economic burden of influenza.
authors.affiliation: Department of Biostatistics, Institute of Basic Medical Sciences, University of Oslo, Oslo, Norway. yiting.xue@medisin.uio.no
authors.forename: Yiting
authors.initials: Y
authors.lastname: Xue
authors.forename: Ivar Sønbø
authors.initials: IS
authors.lastname: Kristiansen
authors.forename: Birgitte Freiesleben
authors.initials: BF
authors.lastname: de Blasio
mesh_terms: D000465:Algorithms; D003365:Costs and Cost Analysis / Q000379:methods; D006801:Humans; D007251:Influenza, Human / Q000150:complications* / Q000191:economics*; D017059:Models, Econometric; D009010:Monte Carlo Method; D009664:Norway; D016012:Poisson Distribution; D018582:Sick Leave / Q000191:economics*