I think this is an overhyped concern. They followed survivors for 8 months, and they had lower antibodies, but they had t cells, and they had enough immunity. They stopped at 8 months, or it would have down a longer effect. T cells id the virus and then the body produces the antibodies. T cell immunity is long lived generally and is why you get chicken pox once, and then almost everyone is immune the rest of their life. Probably the t cells will be around a long time. Half the population had some t cells that could id at least one of two proteins on convid, from having had a common cold from a different corona virus. If half did, then it seems to last many years, since it takes maybe decades for half the people to have the common cold from a corona virus.