Mortality Risk Among Patients With Influenza Illness Admitted to the ICU: A Systematic Review and Meta-Analysis

Despite vaccination programs and available treatments, seasonal influenza carries a large mortality burden, especially in intensive care unit (ICU) settings. Understanding the influenza mortality burden in ICU settings can inform treatment planning and resource allocation. Nonetheless, surveillance data on mortality in ICU-admitted patients are scarce and estimates vary greatly. This systematic literature review (SLR) and meta-analysis investigated all-cause mortality risk among ICU-admitted patients with influenza in Europe.

We included observational studies conducted in Europe that reported mortality among patients ≥ 6 months of age with influenza admitted to the ICU. Studies published between January-2009 and December-2019 were included. Quality was assessed using a modified Newcastle-Ottawa scale. Pooled all-cause mortality risk was calculated as a proportion using a random-effects model with an inverse variance method. A sensitivity analysis was also conducted, including only studies identified as having low risk of bias.

Thirty-seven studies, reporting on 13,616 patients, were included. All-cause mortality ranged from 0% to 67%. The overall pooled mortality risk estimate was 0.24 (95% CI: 0.20, 0.27). Study heterogeneity was high (Cochran's Q test p < 0.01, I² = 93%). The sensitivity analysis using only studies identified as having low risk of bias produced a pooled mortality risk of 0.25 (95%CI: 0.21, 0.29).

These results indicate that approximately a quarter of patients with influenza admitted to the ICU die, reinforcing the need for effective vaccination programs and treatment optimization.