Key Points

The number of hospitalizations for influenza, both with and without admission to intensive care, varied from season to season depending on the circulating influenza subtype(s). Influenza seasons in which the A(H3N2) virus was dominant (2014–2015 and 2016–2017 seasons) were marked by a higher number of hospitalizations. The proportion of hospitalizations requiring admission to intensive care ranged from 6% (2016–2017 season) to 12% (2012–2013 and 2014–2015 seasons).

Hospitalizations for influenza were observed primarily in the extreme age groups (under 20 years and 80 years and older).

The regional incidence of influenza-related hospitalizations requiring admission to intensive care was among the highest in 2016–2017 compared to other regions.

The proportions of influenza-related hospitalizations requiring admission to intensive care were highest among those aged 40–59 and 60–79 (18% and 16%, respectively), while they were less than 5% in the extreme age groups (under 20 and 80 and older).

The average length of stay was 7.3 days for hospitalizations without admission to intensive care and 19.4 days for those with admission to intensive care. The length of stay increased with age.

The case fatality rate among all influenza hospitalizations increased with age, ranging from 1% among those under 40 to 10% among those aged 80 and older.

Among those under 20 years of age, hospital stays were predominantly classified in the Homogeneous Patient Group (HPG) "moderate severity influenza," whereas stays for those aged 80 and older were in the HPG "severe influenza."