Probably the two to three year figure came from people who are concerned about the phenomenon of Antibody Dependent Enhancement.  That is, the immune system is primed to over-react upon exposure to a wild strain variant of a pathogen to which a subject was 'inoculated'.

This ADE (aka 'pathogenic priming') response is unusual, but noted in RSV, Dengue Fever, and coronavirus.  The reason for concern is that a decade ago when last a coronavirus vaccines were attempted lab animals died in startling numbers when exposed to the pathogen they were supposed to be inoculated against.  Development of a coronavirus vaccine stalled.  Or at least it waited for a time when animal trials could be avoided due to an artificially induced panic.

Anyway, natural coronavirus goes around (the world) every few years as the common cold.  Probably the two or three year projections of death for gene therapy trial participants reflects that.

What I suspect will happen is that the common cold will become a death sentence for people who got 'on the vax train' and they will have no choice but to get follow-on 'boosters' because the common cold has gone from a two day runny nose to a likely toe-tag event for them.