2.2 Study definitions
Patients with Flu-p were defined as individuals for whom polymerase
chain reaction (PCR) analyses of respiratory specimens (including
sputum, nasal/nasopharyngeal swabs, bronchial aspirates, and
bronchoalveolar lavage fluid) were positive for influenza viral RNA, and
for whom respiratory symptoms and chest radiographic findings were
consistent with newly emergent chest infiltrates. The decisions to
initiate IMV were taken by the attending physicians, based on the
presence of any of the following intubation criteria: respiratory or
cardiac arrest, respiratory pauses with loss of alertness or gasping for
air, severely impaired consciousness, major agitation inadequately
controlled by sedation, signs of exhaustion, massive aspiration,
inability to manage respiratory secretions appropriately, and
hemodynamic instability without response to fluids and vasoactive
agents. Additionally, patients were also intubated in case of subsequent
worsening of gas exchange or respiratory distress despite supportive
measures [12]. Early neuraminidase inhibitor (NAI) therapy was
defined as the administration of NAI agents within two days of symptom
onset [132]. Systemic corticosteroid treatment was defined by the
administration of one or more systemic corticosteroid doses before
invasive ventilation and after admission. Community-acquired
co-infecting respiratory pathogens were defined as pathogens detected
via standard microbiological techniques (Supplementary Material 2)
within 48 h following admission [14].