This rapid logarithmic increase in antibody titer is the result of the first vaccine (or exposure to virus) inducing memory B cells, so that there are many more cells ready to respond than with the first exposure.

An interesting consideration with the omicron variant is protection from previous infection vs vaccination. Prior to omicron, 2 mRNA vaccine doses were generally more effective than prior infection. Now that the spike protein structure is so different with omicron, antibodies specific for prior spike protein structures are less effective. However, the body makes antibodies to many different viral proteins, and some of these are “ neutralizing”. Antibodies to internal viral proteins with greater structural stability may confer protection in prior infected people that will be missing from vaccination.

Fascinating stuff!