Clinical Characteristics and Outcomes of ICU Patients During the First Post-COVID-19 2023-2024 Influenza Season in The Netherlands.

BACKGROUND: Influenza can cause severe complications, especially in patients with specific risk factors or comorbidities associated with poor outcomes. Some patients are at increased risk of a complicated disease course, including secondary infections, ICU admission, and the need for mechanical ventilation. The first post-COVID-19 seasonal influenza season placed a substantial burden on Dutch ICUs. This study investigates the disease course and outcomes of ICU patients with influenza. METHODS: A retrospective influenza registry study was conducted across 34 Dutch ICUs, including patients aged 18 and older admitted to the ICU with a positive influenza RT-PCR test, between 1 November 2023 and 17 March 2024. Data on demographic information, medical history, clinical symptoms, laboratory and imaging results, parameters of mechanical ventilation, additional treatments, length of hospital stay, and mortality was retrieved from the electronic patient record. RESULTS: A total of 498 patients were included in the study. The median age was 64 (IQR: 55-72) years and 58.8% of the patients were men. The most common comorbidities were cardiovascular disease (34.1%), chronic obstructive pulmonary disease (COPD) (31.5%), and diabetes (22.3%). Bacterial co-infections were present in 37.6% of the patients. Invasive mechanical ventilation (IMV) was necessary in 46.0% of patients, 38.0% of those requiring IMV were treated in prone position. A substantial mortality rate was observed, with an ICU mortality rate of 21.9% and an additional hospital mortality rate of 5.2%. CONCLUSION: This study described the characteristics and course of disease of all patients with laboratory-confirmed influenza infection admitted to one of the 34 participating Dutch ICUs between November 2023 and March 2024. The major findings of this study are the substantial mortality rate, a high proportion of patients with bacterial co-infections, and a significant percentage of patients requiring IMV and prone position ventilation. Finally, patients without comorbidities that were admitted to the ICU with an influenza virus infection showed severe disease parameters but had a lower mortality than patients with comorbidities.