The saline placebo was administered in the same volumes as the active agents in the XAV-19 group, so that neither the patient nor the investigator could differentiate it from XAV-19 by its appearance. Based on a previous pharmacokinetic study (phase 2a) of XAV-19 in patients hospitalized with COVID-19 pneumonia, a single infusion of XAV-19 at Santonin 2?mg/kg was chosen [15]. enrollment was 9 (7C10) days. There was no statistically significant decrease in the cumulative incidence of death or severe respiratory failure through day 15 in the XAV-19 group vs the placebo group (53/199 [26.6%] vs 48/189 [25.4%]; adjusted risk difference, 0.6%; 95% CI, ?6% to 7%; hazard ratio, 1.03; 95% CI, 0.64C1.66; = .90). In the safety population, adverse events were reported in 75.4% of 199 patients in the XAV-19 group and in 76.3% of 190 patients in the placebo group through D29. Conclusions Among patients hospitalized with COVID-19 requiring low-flow oxygen therapy, treatment with a single intravenous dose of XAV-19, compared with placebo, did not show a significant difference in terms of disease progression at day 15. Keywords: SARS-CoV-2, XAV-19, clinical trial, polyclonal glyco-humanized anti-SARS-CoV-2 antibody Coronavirus disease 2019 Santonin (COVID-19) pneumonia in hospitalized patients often leads to acute respiratory distress syndrome and use of high-flow oxygen, mechanical ventilation, and a high rate of death [1]. In the past 2 years, progress has been made to improve management of the most severe forms of COVID-19, including heparin-based thromboprophylaxis [2], remdesivir [3], glucocorticoids [4], interleukin-6 signaling inhibitors [5], selective inhibitor of Janus kinase (JAK) [6], and neutralizing antibodies [7]. Many neutralizing monoclonal antibody treatments have been developed and have been recommended by the World Health Organization for patients at an early stage of COVID-19 to prevent progression to severe or critical disease [7, 8]. Given the resistance of current SARS-CoV-2 variants to monoclonal antibodies, there is a clinical need for new neutralizing antibodies for management of COVID-19 patients. The advantage of polyclonal antibodies over monoclonal antibodies, which bind to nonoverlapping epitopes, is to reduce neutralization escape by SARS-CoV-2 and mutations in the spike gene [9, 10]. Heterologous animal-derived polyclonal antibodies could be an advantageous approach but raise safety concerns related to the risk of serum sickness [11]. XAV-19 is a purified polyclonal immunoglobulin G (IgG) derived from immunization with the receptor binding domain (RBD) of the SARS-CoV-2 spike in CMAH/GGTA1 double-knockout pigs designed to produce polyclonal hJumpy glyco-humanized antibodies, leading to improved tolerability for administration in humans [12]. XAV-19 binds multiple target epitopes on SARS-CoV-2 spike, maintains neutralizing activity against the Alpha, Beta, Gamma, Delta, and Omicron variants of concern, and does not induce escape mutations in SARS-CoV-2 [13]. In a phase 2a study conducted in severe COVID-19 hospitalized patients, a single intravenous perfusion of XAV-19 at 2?mg/kg was safe and maintained plasma XAV-19 concentrations above the expected target neutralization concentration for at least 8 days after infusion (estimated half-life of 11.4 days) [14, 15]. We conducted a multicenter, randomized, double-blind, placebo-controlled phase 2b study to investigate the efficacy of XAV-19 in hospitalized patients with severe COVID-19 who required low-flow oxygen support. METHODS Trial Design and Ethical Considerations The POLYCOR trial was a multicenter, phase 2b, double-blind, placebo-controlled, randomized clinical trial conducted at 34 sites in France. Details of the trial design have been reported previously [14] and are available in the trial protocol and the statistical analysis plan (Supplementary Data 1 & 2). Patient Consent This trial was conducted in accordance with the Good Clinical Practice guidelines of the International Council for Harmonization E6 and the principles of the Declaration of Helsinki. The protocol was reviewed by the French National Agency for Santonin Medicines and Health Products Safety (ANSM MEDMSANAT-2020-12-00243_2020-002574-27, approval 12/28/2020) and approved by the Ethics Committee CPP Ouest VI (Brest, France, approval #20.06.15.31.306, CPP reference 1305, 01/08/2021) and was sponsored by the research department of Nantes University Hospital. Written informed consent was obtained from all.