there is basically a memory based response because it's similar enough to other corona viruses we have seen
This people know by comparing the immune response of a naive person such as a child never exposed to anything to that of an adult. The adult system recruits cells foremost from memory because it recognizes the structures.
Another thing i've learned is that these corona viruses mutate a lot because they don't carry a backup copy of their genes. This causes a lot of errors in their replication.
It would be difficult to find two viruses in a load which are exact 100% replica of eachother and when we are exposed to a viral load then many if not most of those units are not viable for replication.
What people do to determine a new variant is sequencing the genes of a viral load to make an average representation of the genes therein. This is then called a master sequence.
When one master sequence deviates from another by 0.3% or so (?), it is called a new variant of the virus.
SARS-CoV-2 is genetically similar to SARS-CoV. SARS-CoV is the nearest human coronavirus relative of SARS-CoV-2 with an 86.85% similarity, which is reflected in their similar names. The currently known closest relative of SARS-CoV-2 is the bat coronavirus RaTG13 which has 96.75% similarity.
https://www.news-medical.net/health/How ... S-CoV.aspx