Inclusion Criteria:  Group 1 (Covid-19 early pneumonia)  Age > 18 Positive Sars-CoV 2 nasal swab interstitial pneumonia at either CT scan or chest X-ray Respiratory failure requiring CPAP for less than 48 hours FiO2 ≤0.5 and CPAP≤10 cmH2O  Group 2 (Covid-19 severe pneumonia)  Age > 18 Positive Sars-CoV 2 nasal swab interstitial pneumonia at either CT scan or chest X-ray Respiratory failure requiring CPAP  Signs of severity with CPAP 10 cmH2O and FiO2 0.5: pulse oximetry (SpO2) ≤ 93% associated to either:  Dyspnea Two or more signs of increased respiratory effort (respiratory rate ≥25 bpm, use of accessory inspiratory muscles , tirage, intercostal space depression, nasal flaring, expiratory abdominal efforts, PaCO2 < 35)  Group 3 (Non Covid-19 pneumonia)  Age > 18 Negative Sars-CoV 2 nasal swab CT scan or chest X-ray non compatible with Covid-19 associated pneumonia Respiratory failure requiring CPAP  Exclusion Criteria:  Group 1 (Covid-19 early pneumonia)  Concomitant chronic pulmonary disease Chronic heart failure New York Heart Association (NYHA) 3-4 Bacterial pulmonary associated infection (diagnosed or suspected) Pulmonary embolism Acute cardiogenic pulmonary edema  Signs of severity with CPAP 10 cmH2O and FiO2 0.5: SpO2≤ 93% associated to either:  Dyspnea Two or more signs of increased respiratory effort (respiratory rate ≥25 bpm, use of accessory inspiratory muscles , tirage, intercostal space depression, nasal flaring, expiratory abdominal efforts, PaCO2 < 35) At least one sign of respiratory fatigue/decompensation (pH<7.30 with PaCO2 >45, respiratory rate <15 bpm, paradoxal abdominal breathing, mental status alteration)  Group 2 (Covid-19 severe pneumonia)  Concomitant chronic pulmonary disease Chronic heart failure NYHA 3-4 Bacterial pulmonary associated infection (diagnosed or suspected) Pulmonary embolism Acute cardiogenic pulmonary edema At least one sign of respiratory fatigue/decompensation (pH<7.30 with PaCO2 >45, respiratory rate <15 bpm, paradoxal abdominal breathing, mental status alteration)  Group 3 (Non Covid-19 pneumonia)  Concomitant chronic pulmonary disease Chronic heart failure NYHA 3-4 Bacterial pulmonary associated infection (diagnosed or suspected) Pulmonary embolism Acute cardiogenic pulmonary edema At least one sign of respiratory fatigue/decompensation (pH<7.30 with PaCO2 >45, respiratory rate <15 bpm, paradoxal abdominal breathing, mental status alteration)