Kim 2011.

Methods	Multicentre retrospective cohort/case‐control
Participants	Country: Korea Setting: ICU Number of sites: 28 Number of individuals: 245 Inclusion criteria: age >= 15 years; presence of critical illness defined as i) admitted to ICU or required mechanical ventilation (invasive or non‐invasive), or ii) had PaO2/FiO2 less than 300 mmHg, or iii) required intravenous infusion of an inotropic or vasoconstrictive medication) Definition of influenza: laboratory confirmed Influenza type: 2009 influenza A H1N1 virus (H1N1pdm09) Mean age (years): no CS group 54.1 (± 19.3); CS group 56.9 (± 17.2) Male sex: no CS group 73 (53.0); CS group 61 (57.0) Comorbid illnesses: Asthma: CS 10 (9); no CS 9 (7) COPD: CS 14 (13); no CS 6 (4) Solid tumour: CS 30 (28); no CS 19 (14) Disease severity: mean APACHE II: no CS group 17.5 (± 8.5); CS group 21.2 (± 7.7); P = 0.001
Interventions	Groups: CS treatment (n = 107) versus no CS treatment (n = 138) Dose: median prednisolone equivalent 75 mg/day (IQR 50 to 81 mg/day) Duration of treatment: median 6 (IQR 6 to 13) days Antibiotics: CS group 107 (100.0); no CS group 136 (99.0) Antivirals: CS group 44 (41.0); no CS group 68 (49.0)
Outcomes	Mortality: 14‐day, 30‐day, and 90‐day mortality reported. Adjusted estimates presented for 90‐day mortality (following adjustment for age, SOFA score, mechanical ventilation, lymphocyte count, and propensity score) Unadjusted estimates given for mechanical ventilation and length of stay. Adverse events: secondary bacterial infections
Risk of bias (Newcastle‐Ottawa Scale)	Mortality Selection domain score (max 4): 4 Comparability domain score (max 2): 2 Outcome domain score (max 3): 3 Mechanical ventilation Selection domain score (max 4): 4 Comparability domain score (max 2): 0 Outcome domain score (max 3): 3 Length of stay Selection domain score (max 4): 4 Comparability domain score (max 2): 0 Outcome domain score (max 3): 3 Hospital‐acquired infection Selection domain score (max 4): 4 Comparability domain score (max 2): 0 Outcome domain score (max 3): 3
Notes	Adjusted estimates for 90‐day mortality presented following logistical regression in a cohort study. Separate estimates given in a propensity matched case‐control study.