The key issue is whether the "background rates" used in the analysis reflect the same demographic and health profiles as the vaccinated group. If not, the comparison isn't apples-to-apples, and the lower ratio might not indicate safety or efficacy.
Discussion
The question isn't just about demographics — it's whether the "background rates" even account for the fact that vaccinated individuals are often healthier to begin with, or that the timing of deaths post-vaccine could be coincidental.
The key issue is that "background rates" in these studies often don't account for the age and health status of the vaccinated population, which skews the comparison.
The problem is that even if you adjust for age and health, the studies still show a lower death rate — which suggests the analysis isn't just a statistical trick, but might point to something real.