[User23]

This statement assumes that death is the only bad outcome from COVID that sufficient Vitamin D3 levels prevent.

Death is useful as a metric because, unlike other health outcomes, particularly quality of life ones, there's no element of subjectivity.

[rossdavidh]

Another useful aspect of death as a metric, is that it is the most widely reported (as compared to, say, maximum fever, amount of ventilation required, days in hospital, lasting anosmia, etc. etc.)

[simion314]

You could measure days of hospitalization too, I would suggest both vaccine and Vitamin supplements if there are no side-effects.

[wrycoder]

It appears that with COVID, the proximate cause of death is often mis-coded, for financial reasons.