Influenza-associated hospitalisations in Finland from 1996 to 2010: unexpected age-specific burden during the influenza A(H1N1)pdm09 pandemic from 2009 to 2010

To assess the burden of influenza on the Finnish healthcare system, we analysed hospitalisations during 1996-2010 using the International Classification of Diseases codes potentially related to influenza and its complications from the national hospital discharge registry. To compare the influenza A(H1N1)pdm09 pandemic with previous influenza seasons in 1996-2009, we calculated hospitalisation rates by age-and diagnostic groups. We built a negative binomial regression model based on times series analysis to assess the impact of the pandemic. Influenza-associated hospitalisation rates were higher during the pandemic compared to pre-pandemic influenza seasons for 5-24 year-olds (incidence rate ratio (IRR): 1.52, 95% confidence interval (CI): 1.44-1.60) and 25-64 year-olds (IRR: 1.33, 95% CI: 1.29-1.36), but did not differ for persons aged >= 65 years (IRR: 0.98, 95% CI: 0.97-1.00). Hospitalisation rates exceeded the upper limit of the prediction line by 177% in 5-24 year-olds, 66% in 0-4 year-olds and 57% in 25-64 year-olds. During the influenza season of 2003./04, all age groups had higher-than-expected hospitalisation rates, whereas other seasonal peaks were only notable among persons aged >= 65 years. These age-specific differences in the hospital burden underscore the importance of the continuous surveillance of hospitalisations in order to evaluate immunisation priorities for seasonal influenza and pandemic preparedness including use of antiviral medication.