produce a spike protein common to all of the SARS-CoV-2 virus strains.
Then our immune system develops a multifaceted response to this harmless spike protein and retains a memory so that when the real SARS-CoV-2 virus (with its spike protein) shows up, our immune system is primed and ready to attack the real virus.
It would seem unlikely that the stalk proteins would change significantly since it is a necessary part which allows the virus to attach to the cell membrane at the ACE-2 receptor site. If the spike protein changes (because of a significant mutation) it might cause the virus to not attach and the virus would not survive.