Early High-Titre Convalescent Plasma in Clinically Vulnerable Individuals With Mild COVID-19

* Research Question: Does convalescent plasma (CCP) collected from donors who have recovered from COVID-19 and who have a very high titre of anti-SARS-CoV-2 antibodies reduce the risk of hospitalisation (for COVID-19) or death in patients with early symptoms of acute COVID-19 who are vulnerable to this disease compared to standard of care? * Study product: Very high antibody titre COVID-19 convalescent plasma collected more than 15 days after end of symptoms in COVID-19 patients who also had received at least one dose of a SARS-CoV-2 vaccine. * Methodology: Multicentre, randomised, open-label, adaptive superiority trial: COVID-19 very high neutralizing Ab titre convalescent plasma vs standard care in 2 cohorts of vulnerable patients (cohort 1: elderly (≥ 70 years) and younger with comorbidities, cohort 2: immunosuppressed patients). * Study phase: Phase 3 * Intervention: Two units of high antibody titre COVID-19 convalescent plasma to individuals randomised to the intervention group, 2 units from 2 different donors, preferably transfused on the same day. Plasma provided by convalescent vaccinated donors with a minimum antibody titre of 1:640 against delta variant (B1.617.2) or antibody concentration \>=4.000 BAU/ml in the QuantiVac anti-SARS-CoV-2 IgG ELISA or \>=20.000 U/ml in the Elecsys anti-SARS-CoV-2 CLIA * Randomisation: 1:1 (standard of care + convalescent plasma vs. standard of care) stratified by centre (cohorts 1 and 2)