We collected dates of hospital and ICU admission, and of MV onset; demographics; risk factors for influenza A; actual weight; height; severity of illness (Acute Physiology And Chronic Health Evaluation II, APACHE II), organ failures (Sequential Organ Failure Assessment, SOFA); type of MV used, as noninvasive (NIV) and invasive; and date of intubation. Ideal body weight (IBW, ml/kg) and body mass index (BMI) were calculated; obesity was defined as a BMI > 30.