I provide links below to a news article and the Red Cross itself. Both say that the vaccinated can give blood and plasma, but not convalescent plasma. Convalescent plasma is given to COVID patients as a treatment. According to the KOMU news article: “The Red Cross said vaccinated people can’t donate plasma because the shot counteracts existing antibodies.”
Why would this be the case? I do not know the answer to this question, but it reminds me of something Dr. Bossche expressed in this interview as a concern of the gene therapeutic vaccine–that the gene therapeutic instructions override one’s natural or innate immune response.