Given that a lot of the COVID morbidity and mortality is the cytokine storm rather than the infection itself, I wouldn't consider it a priori clear that Vitamin D helps, is neutral, or harms patients who get COVID.

The result isn't sufficient to answer the question. It establishes, generally, that Vitamin D deficient people are more likely to get COVID. It is not able to establish whether these suffer a common cause which affects both Vitamin D levels and COVID (even simple geography or SES would matter -- poor people are more likely to be vitamin deficient, and also more likely to work and live in a manner that exposes them to COVID). The included studies in the meta-analysis seem inconsistent about whether they attempt to adjust (in a "selection on observables" sense) for confounders, but only a few even mention confounders. For that reason I am not sure the use of the causal language "impact of... on" in the title is justified.

I would consider this a lead that is worth investigating, but haven't really adjusted my very diffuse priors on the basis of this MA.