Respiratory involvement of SARS-CoV2 leads to acute respiratory distress syndrome (ARDS) and significant immunosuppression (lymphopenia) exposing patients to long ventilation duration and late mortality linked to the acquisition of nosocomial infections. Lymphopenia characteristic of severe forms of ARDS secondary to SARS-CoV2 infection may be linked to expansion of MDSCs and arginine depletion of lymphocytes. Severe forms of COVID-19 pneumonitis are marked by persistent ARDS with acquisition of nosocomial infections as well as by prolonged lymphocytic dysfunction associated with the emergence of MDSC. It has been found in intensive care patients hypoargininaemia, associated with the persistence of organ dysfunction (evaluated by the SOFA score), the occurrence of nosocomial infections and mortality. Also, it has been demonstrated that in these patients, the enteral administration of ARG was not deleterious and increased the synthesis of ornithine, suggesting a preferential use of ARG by the arginase route, without significant increase in argininaemia nor effect on immune functions. L-citrulline (CIT), an endogenous precursor of ARG, is an interesting alternative to increase the availability of ARG. Recent data demonstrate that the administration of CIT in intensive care is not deleterious and that it very significantly reduces mortality in an animal model of sepsis, corrects hypoargininemia, with convincing data on immunological parameters such as lymphopenia, which is associated with mortality, organ dysfunction and the occurrence of nosocomial infections. The availability of ARG directly impacts the mitochondrial metabolism of T lymphocytes and their function. The hypothesis is therefore that CIT supplementation is more effective than the administration of ARG to correct hypoargininaemia, decrease lymphocyte dysfunction, correct immunosuppression and organ dysfunction in septic patients admitted to intensive care. The main objective is to show that, in patients hospitalized in intensive care for ARDS secondary to COVID-19 pneumonia, the group of patients receiving L-citrulline for 7 days, compared to the group receiving placebo, has a score of organ failure decreased on D7 (evaluated by the SOFA score) or by the last known SOFA score if the patient has died or been resuscitated.