In the research community there are ongoing efforts to build biobanks of patient samples, and to test the serum antibodies to see whether these antibodies are neutralizing antibodies; you require these. For this we have many research institutions that are building up infrastructure to test and to come to a conclusion as to when these neutralizing antibodies appear and whether they can be used. There is still upstream research to be done. I'm not certain that you can take plasma from anyone who had the disease and use it as a therapy. Some research has to go on before you can do this.
On April 30th, 2020. See this statement in context.