This just rules our using ZJ01. There are other attenuated strains out there that will have deletion mutations that can’t mutate back. We need to go find them.
If I understand correctly, antigens identify the virus by its S proteins. Would we be able to use the same methods <a href='https://www.nature.com/articles/nm.3985'>as in this paper</a> to replace the S protein in a harmless cold-like coronavirus with the S protein of SARS-CoV-2, and would then the resulting immunity defend against SARS-CoV-2?
Send people infected with the harmless version around, after lockdown ends in a month or two, to crowded places?
Would greater virulence be a desirable quality in our reduced strain?
I don’t think we will get to that point. If we collect good data from the natural spread of any attenuated strain then I think the regulatory authorities will allow its use on the basis of this data.