Hmm, it's true that pandemic stress can lower mental health. But the study showed that vaccinated covid survivors have lower risk of mental health issues compared to unvaccinated covid survivors.

If this result is confounded by pandemic stress, it would mean that pandemic stress somehow reduced vaccine uptake, which could be true in some cases. This is why we can't really infer cause-and-effect from non-randomized observational studies due to the inevitable presence of confounding factors, both known and unknown.