Burden of medically attended influenza in Norway 2008-2017.

BACKGROUND: The burden of influenza in Norway remains uncertain, and data on seasonal variations and differences by age groups are needed.

OBJECTIVE: To describe number of patients diagnosed with influenza in Norway each season and the number treated in primary or specialist health care by age. Further, to compare the burden of seasonal influenza with the 2009-2010 pandemic outbreak.

METHODS: We used Norwegian national health registries and identified all patients diagnosed with influenza from 2008 to 2017. We calculated seasonal rates, compared hospitalized patients with patients in primary care and compared seasonal influenza with the 2009-2010 pandemic outbreak.

RESULTS: Each season, on average 1.7% of the population were diagnosed with influenza in primary care, the average rate of hospitalization was 48 per 100 000 population while the average number of hospitalized patients each season was nearly 2500. The number of hospitalized influenza patients ranged from 579 in 2008-2009 to 4973 in 2016-2017. Rates in primary care were highest among young adults while hospitalization rates were highest in patients 80 years and older and in children below 5 years. The majority of in-hospital deaths were in patients 70 years and older. Fewer patients were hospitalized during the 2009-2010 pandemic than in seasonal outbreaks, but during the pandemic, more people in the younger age groups were hospitalized and fatal cases were younger.

CONCLUSION: Influenza causes a substantial burden in primary care and hospitals. In non-pandemic seasons, people above 80 years have the highest risk of influenza hospitalization and death.