I think they know there will be mutant strains out there totally resistant to the vaccine models we have and potentially much more deadly, that could wipe out millions
To set your mind at rest, this is really incredibly unlikely. I really don't think anyone knows any such thing, they just got a bit bitten on the arse by the second wave due to a somewhat more contagious variant and are being very cautious now.
I also mentioned that within a year we'll have a vaccine that works against all variants as it attacks the core viral structure not the spike protein, which is what mutates to avoid detection by the immune system.