Key Points

Total number of hospitalizations for influenza, both with and without admission to intensive care, and incidence rates have been rising since 2014 in the Hauts-de-France region. Incidence rates rose from 9 per 100,000 inhabitants (541 hospitalizations) during the 2013–2014 season to 33 per 100,000 (1,992 hospitalizations) during the 2016–2017 season. Flu seasons in which the A(H3N2) virus was the predominant strain (2014–2015 and 2016–2017) were marked by a higher number of hospitalizations requiring intensive care and higher incidence rates of such hospitalizations.

Flu-related hospitalizations in Hauts-de-France are observed primarily among those under 20 years of age, with higher hospitalization rates than at the national level in this age group.

The proportions of influenza-related hospitalizations requiring intensive care were highest in the middle age groups (40–79 years), ranging from 17% to 19%, while they were low (less than 7%) in the other extreme age groups (including those under 20 and 80 years and older).

In the Hauts-de-France region, the average length of stay increased with age for hospitalizations without admission to intensive care and for all hospitalizations. The average length of stay was 5.6 days for hospitalizations without admission to intensive care and 18.2 days for those with admission to intensive care.

The case fatality rate among all influenza hospitalizations increased with age, ranging from 1% among those under 20 to 10% among those aged 80 and older.

Among those under 20 years of age, most hospital stays were 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."