Just a thought here based on my recall of Pfizer EUA data and maybe some from whatever approval/authorization A-Z had in UK. The original Pfizer-BioNTech vax produced average peak antibody titers of something like 19,000 compared to average peak titers of 490 from natural infection. Pfizer also had average antibody titers of 4x that of natural infection over the course of the EUA trial.

Conversely A-Z was something like 670 peak titers. This implies that mRNA shots result in much more pseudo spike protein circulating in the body.

Could it be the very high levels of circulating spike, especially if the high levels are chronic due to repeated boosting, that is driving the IgG4 response?