Incidence and costs of hospitalized adult influenza patients in The Netherlands: a retrospective observational study.

Influenza virus infections cause a high disease and economic burden during seasonal epidemics. However, there is still a need for reliable disease burden estimates to provide a more detailed picture of the impact of influenza. Therefore, the objectives of this study is to estimate the incidence of hospitalisation for influenza virus infection and associated hospitalisation costs in adult patients in the Netherlands during two consecutive influenza seasons. We conducted a retrospective study in adult patients with a laboratory confirmed influenza virus infection in three Dutch hospitals during respiratory seasons 2014–2015 and 2015–2016. Incidence was calculated as the weekly number of hospitalised influenza patients divided by the total population in the catchment populations of the three hospitals. Arithmetic mean hospitalisation costs per patient were estimated and included costs for emergency department consultation, diagnostics, general ward and/or intensive care unit admission, isolation, antibiotic and/or antiviral treatment. These hospitalisation costs were extrapolated to national level and expressed in 2017 euros. The study population consisted of 380 hospitalised adult influenza patients. The seasonal cumulative incidence was 3.5 cases per 10,000 persons in respiratory season 2014–2015, compared to 1.8 cases per 10,000 persons in 2015–2016. The arithmetic mean hospitalisation cost per influenza patient was €6128 (95% CI €4934–€7737) per patient in 2014–2015 and €8280 (95% CI €6254–€10,665) in 2015–2016, potentially reaching total hospitalisation costs of €28 million in 2014–2015 and €20 million in 2015–2016. Influenza virus infections lead to 1.8–3.5 hospitalised patients per 10,000 persons, with mean hospitalisation costs of €6100–€8300 per adult patient, resulting in 20–28 million euros annually in The Netherlands. The highest arithmetic mean hospitalisation costs per patient were found in the 45–64 year age group. These influenza burden estimates could be used for future influenza cost-effectiveness and impact studies.