Sargramostim is a man-made form of the naturally-occurring protein GM-CSF.<h4>Trial design</h4>A phase 4 academic, prospective, 2 arm (1:1 ratio), randomized, open-label, controlled trial.<h4>Participants</h4>Patients aged 18-80 years admitted to specialized COVID-19 wards in 5 Belgian hospitals with recent (< 2 weeks prior to randomization) confirmed COVID-19 infection and acute respiratory failure defined as a PaO2/FiO2 below 350 mmHg or SpO2 below 93% on minimal 2 L/min supplemental oxygen. Here, CSF2 is linked to COVID-19.