Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has spread globally since December 2019 causing a worldwide pandemic of coronavirus disease 2019 (COVID-19). Systemic hyper-inflammation is a hallmark of severe stages of COVID-19 leading to acute lung injury (ALI), need for mechanical ventilation and ultimately death. The mechanism responsible for virus-induced hyper-activation of the host immune system remains to be fully elucidated. Unregulated complement system activation induced by CoVs plays a crucial role in the pathogenesis of acute lung injury in COVID-19. Similarly, involvement of the kinin-kallikrein system in capillary leakage and subsequent pulmonary angioedema has been suspected. In particular, a reduced activity of angiotensin converting enzyme 2 (ACE2) caused by SARS-CoV-2 leads to a relative abundance of bradykinin degradation products and local pulmonary edema. Lastly, over-activation of the contact activation system may be involved in the observed thromboinflammation. C1 esterase inhibitor (C1INH) is a potent inhibitor of these three plasmatic cascades and has been shown to reduce pulmonary inflammation and death in a CoV mouse model and in human studies of severe sepsis. Aims: To assess the safety and efficacy of recombinant C1INH (conestat alfa, Ruconest®) in addition to standard of care (SOC) in patients hospitalized with severe SARS-CoV-2 compared to SOC only.Hypothesis: Administration of conestat alfa for 72 hours in addition to SOC in patients with severe SARS-CoV-2 pneumonia is safe and associated with a reduced clinical severity on day 7 and a lower risk of disease progression to mechanical ventilation compared to SOC.Methodology: The PROTECT-COVID-19 trial is a randomized, open-label, parallel-group, controlled, multi-center clinical trial. Consecutive patients admitted with COVID-19 not requiring intensive care support will be randomized in a 2:1 ratio to treatment with conestat alfa for 72 hours in addition to SOC or SOC only. The primary endpoint will be the disease severity on the 7-point Ordinal WHO scale on day 7. Secondary endpoints include time to clinical improvement, the proportion of participants alive and not having required invasive or non-invasive ventilation and the incidence of ALI within 14 days after enrolment. Treatment safety and the activity of the above mentioned plasmatic cascades will be assessed. For a 2:1-randomization, a nonparametric analysis by the stratified logrank-test, and an adaptive group sequential analysis, the overall sample size is estimated as 120 = 80 + 40. Two interim analyses after 40 and 80 patients are planned according to the Pocock adjusted levels ap = 0.0221.Potential significance: Targeting multiple inflammatory cascades with conestat alfa early during COVID-19 is an innovative approach to prevent disease progression by ameliorating excessive (thrombo-) inflammation. Interventions that prevent deterioration and mechanical ventilation in COVID-19 are highly desired in a pandemic situation with limited ICU and ventilation support capacity. If treatment with conesta alfa proves to ameliorate pulmonary inflammation and injury, this may be relevant not only on an individual but also on a population level.