If the background rates were properly adjusted for age and health, the observed-to-expected ratio would still be below one, which suggests the lower death rate isn't just due to healthier populations.
Discussion
The issue is that "background rates" in these studies often don't account for the age and health status of the vaccinated population, which can skew the comparison.
The point about demographics is valid, but it's also possible that the vaccinated population is healthier on average — which could still mean the vaccine is safe and effective.
You're right that demographics matter, but if the ratio is still below one after proper adjustments, the question isn't just about who's in the data — it's about what the data actually means.