Is there a role for vitamin D in preventing or treating #COVID19? Our team @PublicHealthON reviewed 54 studies on this topic. Here's the rapid review:
https://www.publichealthontario.ca/-/media/documents/ncov/he/2021/02/covid-19-rapid-review-vitamin-d.pdf?la=en more detail in below

In the pre-COVID days, a large meta-analysis of individual patient data found vitamin D was associated with a modest reduction in the risk of respiratory tract infection (adjusted odds ratio 0.88, 95%CI 0.81 to 0.96). This benefit was more pronounced if vitamin D levels were low.

But is there a protective effect for COVID-19? Forty three observational studies found that lower vitamin D LEVELS are associated with a greater risk of COVID-19 infection and mortality. However there is ++ risk for confounding.

What about vitamin D SUPPLEMENTATION? Ten observational studies again found a protective effect! But ++ risk for confounding, especially the healthy user effect - tendency for patients who receive 1 preventative therapy to receive other beneficial strategies (eg. diet, exercise)

So the answer must come from randomized controlled trials? But There are only 3 RCTs evaluating vitamin D and they are only for TREATMENT of COVID-19. To top it off the largest RCT including 240 patients found no benefit in LOS, ICU admission or mortality. https://www.medrxiv.org/content/10.1101/2020.11.16.20232397v1

We did a meta-analysis of the available mortality data and found a numerically lower but non-statistically significant decrease in mortality with vitamin D supplementation. Randomized trial data however are inconclusive.

So- the jury is still out...
But http://ClinicalTrials.gov  indicates there are at least 30 interventional trials underway evaluating vitamin D and COVID-19 prevention (n=9) or treatment (n=21).

In the meantime, vitamin D is relatively safe at recommended doses for bone health between 800 and 2000 IU dailly, and many of us have insufficient levels anyway. So taking vitamin D is reasonable. But will it prevent COVID-19?