Early, very high-titre convalescent plasma therapy in clinically vulnerable individuals with mild COVID-19 (COVIC-19): protocol for a randomised, open-label trial by Desmarets, M, Hoffmann, S, Vauchy, C, Rijnders, BJA, Toussirot, E, Durrbach, A, Körper, S, Schrezenmeier, E, van der Schoot, CE, Harvala, H, Brunotte, G, Appl, T, Seifried, E, Tiberghien, P, Bradshaw, D, Roberts, DJ, Estcourt, LJ, Schrezenmeier, H