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.
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.