The Broadening of Memory Cells After a Third Boost

It's hard to say whether the broadening comes from re-affinity maturation of the memory or it comes from just harvesting a lot of the memory that was sitting there but was not in the serum at that point. And I think one way that viruses escape is by making mutations that block certain antibodies from binding. Better antibodies with higher, larger, higher affinity surfaces against the antigen are harder to knock off with one mutation.