Abstract: A method of purifying Extracellular Vesicle Capture by AnTibody of CHoice and Enzymatic Release (EV-CATCHER), designed for high-throughput analysis of low-abundance small-RNA cargos by next-generation sequencing, and use of this method for preparing purified populations of biological particles or cells from a biological sample for in vitro evaluation of a patient's risk of developing and for treating a severe viral infection; for enhancing therapeutic effectiveness of convalescent plasma therapy in a patient at risk for a severe coronavirus infection, and for treating a patient with a severe coronavirus infection, are described.