We can see that in data corrected for prevalence of vaccination, the Covid-19 deaths are higher in the unvaccinated group compared to the three vaccinated groups.
There appears to be mortality benefit from one or two vaccinations, but the benefit is lost in those with three vaccinations.
My understanding of this is that each vaccination creates an intense immune stimulation, its very purpose. However it is known that immune activation comsumes the essential Vitamin D, a molecule of which can be used only once and then it is irreversibly de-activated. It follows, but is as yet untested or unpublished, that successive vaccinations will consume Vitamin D and deficiency will result. Reduction of immunity will follow, including response to further vaccinations.