OBJECTIVES: Convalescent plasma (CP) treatment of COVID-19 has shown significant therapeutic effect only when administered early. We investigated the importance of patient and CP seroprofiles on treatment outcome in REMAP-CAP CP trial. METHODS: We evaluated neutralising antibodies (nAb), anti-spike (S) IgM, IgG, IgG avidity, IgG fucosylation and respiratory viral loads in a sub-set of patients (n=80) and controls (n=51) before and after transfusion, comparing them to those in the CP units (n=157) they received. RESULTS: Most patients were SARS-CoV-2 seropositive pre-transfusion (72% nAb
89% S-IgG seropositivity). The majority (80%) had higher pre-transfusion S-IgG levels (median 1.7×10 CONCLUSIONS: Based on our data, increased S-antibody levels linked to transfused CP were only observed in pre-seroconversion or immunodeficient patients lacking their own SARS-CoV-2 antibodies, representing the groups where CP treatment has previously shown most benefit.