Safety and Efficacy of AON-D21 in Severe Community-Acquired Pneumonia.

Inclusion Criteria: * Community-acquired pneumonia, confirmed or suspected of bacterial or viral origin. * Admitted to an ICU (or similar unit). * Requiring respiratory support by HFO ≥ 30 L/min with FiO2 ≥ 30% or NIV or IMV or ECMO. * CRP ≥ 50 mg/L. * PaO2/FiO2 ratio ≤ 150 mmHg. * Treatment initiation no more than 48 h after initiation of respiratory support (HFO ≥ 30 L/min with FiO2 ≥ 30%, NIV, IMV or ECMO). * Written informed consent. * Age ≥ 18 years to ≤ 85 years. * Body mass index ≥ 17.5 kg/m² and ≤ 40 kg/m². * For female participants of childbearing potential, agreement to use dual methods of contraception until Day 60. * For male participants with female partners of childbearing potential, agreement to use barrier method of contraception until Day 60 and to refrain from donating sperm during the study and for 3 months after the last infusion. Exclusion Criteria: * Refractory septic shock. * Not expected to survive 72 hours. * Hospital-acquired or ventilator-associated pneumonia or known or suspected pneumonia due to aspiration or other physical injury or trauma or tuberculosis. * Known or suspected hypersensitivity to AON-D21 or any components of the formulation used (e.g., PEG, mannitol or EDTA) or a history of clinically relevant allergy requiring continuous treatment, or of anaphylaxis. * Known fibrotic lung disease, bronchiectasis or any other known severe chronic respiratory disease. * Active malignant disease. * Factors other than a pathogen suspected or confirmed to be causative for the respiratory insufficiency. * Hepatocellular injury defined by an ALT or AST value ≥ 3 times the ULN. Known acute or chronic liver disease with Child-Pugh C (See Appendix 13.6.2). * Any medical disease or condition that, in the opinion of the investigator(s), compromises the participant's safety or compromises the interpretation of the results. * Receiving chronic immunosuppressive therapy in relevant doses. * Known immunodeficiency disease/condition. * Nursing and pregnant women (defined as the state after conception until the termination of gestation, screened in all women of child-bearing potential with a chorionic gonadotrophin (hCG) blood test (local laboratory). * Current or recent participation in an investigational trial. * Systemic treatment with any complement inhibitor. * Known complement deficiency. * Unlikely to remain at the investigational site beyond 96 h.