Key points

• The total number of hospitalizations for influenza, both with and without admission to the ICU, varied from season to season depending on the circulating influenza subtype(s). Flu seasons in which the A(H3N2) virus was the predominant strain (2014–2015 and 2016–2017) were characterized by a higher number of hospitalizations. The proportion of hospitalizations requiring admission to the ICU ranged from 11% (2016–2017 season) to 13% (2013–2014 season).

• The incidence rates of all influenza-related hospitalizations were higher among people aged 80 and older, regardless of the season.

• The proportions of influenza-related hospitalizations requiring admission to the ICU were highest in the middle age groups (40–79 years), ranging from 22% to 23%, while they were low (≤10%) in the extreme age groups (under 20 years and 80 years and older).

• The average length of stay was 6.8 days for hospitalizations without admission to intensive care and 17.6 days for those with admission to intensive care. The length of stay increased with age.

• The case fatality rate among all hospitalizations for influenza 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 a Homogeneous Patient Group (HPG) for “moderate-severity influenza,” whereas among those 80 years and older, the majority of stays were classified in an HPG for “high-severity influenza.”