Key points

The total number of hospitalizations for influenza, with and without admission to the intensive care unit, varied from season to season depending on the circulating influenza subtypes. Flu seasons in which the A(H3N2) virus was the predominant strain (2014–2015 and 2016–2017 seasons) were marked by a higher number of hospitalizations for influenza. The proportion of hospitalizations requiring admission to intensive care ranged from 9% (2016–2017 season) to 14% (2013–2014 season).

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

The proportions of influenza-related hospitalizations requiring admission to the ICU were highest in the intermediate age groups (40–79 years), ranging from 16% to 22%, while they were low in the other extreme age groups, particularly among those under 20 years of age (2%).

The average length of stay was 7.5 days for hospitalizations without admission to intensive care and 20 days for those requiring intensive care. The length of stay increased with age.

The case fatality rate among all hospitalizations increased with age, ranging from 0.2% among those under 20 to 11% among those aged 80 and older.

Among those under 20 years of age, hospitalizations were predominantly classified in a Homogeneous Patient Group (HPG) for "moderate-severity influenza"; among those 80 years and older, the majority of stays were classified in an HPG for "severe influenza."