[pygy_]

Another factor that may play a role: the people who chose not to take the vaccine may be prone to taking bad decisions more broadly, leading to a higher mortality rate.

[oldgradstudent]

If true, that means the groups are different in many other aspects other than the vaccine.

The study does not control for the differences. No causality can be inferred.

[disgruntledphd2]

I mean, it's plausibly something around trust in medical authorities which correlates with both vaccine and other treatment refusal.