Prone position (placing the patient on his abdomen) has been shown to be an effective intervention to decrease mortality in adults connected to mechanical ventilation for moderate to severe Acute Respiratory Distress Syndrome (ARDS). Patients may require one or more sessions of prone position. However, the optimal duration of prone sessions is unknown. The goal of this clinical trial is to learn if applying prone position in prolonged sessions (\> 48 hours - prolonged prone position) is more effective than applying it in daily sessions (16 to 24 hours - intermittent prone position). The trial will also learn about the safety of prolonged prone position compared to intermittent prone position. The main questions it aims to answer are: * Does prolonged prone position increase survival compared to intermittent prone position in participants with moderate to severe ARDS ? * How does prolonged prone position compare to intermittent prone position in terms of medical problems associated to prone position ? Researchers will compare prolonged versus intermittent prone position to see which approach is better to treat moderate to severe ARDS. Participants will: * Receive prone position either in prolonged (\> 48 hours) or daily (16 to 24 hours) sessions during the first 7 days * Be followed for up to 90 days to assess their clinical evolution