The relative clinical impact of 2009 pandemic influenza A (H1N1) in the community compared to seasonal influenza in the Netherlands was most marked among 5-14 year olds.

Background: So far, most pandemic influenza reports were based on case studies focusing on severe disease. For public health policy, it is essential to consider the overall impact of the pandemic, including mild diseases. Objectives: The aim of our study is to gain insight into the epidemiology of 2009 pandemic influenza in the community and to estimate the relative impact of pandemic compared to seasonal influenza. Methods: The relative impact of pandemic influenza in the general population was assessed as the influenza-like illness (ILI) incidence during the pandemic season compared with that during regular seasons. Influenza-like illness incidences and virus diagnostics were derived from continuous sentinel surveillance systems. The incidence of hospital admissions, based on the mandatory notification of pandemic influenza, was used to relate the impact of severe disease to that in the community. Results: The overall incidence of general practitioners-attended ILI was 96 consultations per 10 000 persons. Highest incidences were reported in children and lowest in persons aged ≥65 years. For 5-14 year olds, the incidence during the pandemic was higher than during all preceding seasons. Samples originating from 5 to 19 year olds were statistically significant more often positive for pandemic influenza A (H1N1) 2009 virus as compared with samples from 0 to 4 year olds. Moreover, the incidence of hospital admission owing to pandemic influenza was highest in the youngest children. Conclusions: Our study showed that while the absolute incidences of 2009 pandemic influenza were highest in children aged 0-4 years, the relative clinical impact in the community compared to seasonal influenza in previous years was most noticeable in healthy children 5-14 years of age. (aut. ref.