Direct research on vitamin D and Covid

A Mendelian randomisation analysis of excess COVID-19 mortality of African-Americans in the US suggests that vitamin D is a risk factor for Covid Mortality
A review of the evidende finds that supplementing with vitamin D in accordance with government guidelines is a good idea, but there is no evidence to support supplementation rates higher than 4,000 IU/day

A re-analysis of 107 Swiss blood samples found that PCA positive patients had 25-hydroxyvitamin D concentrations half that of PCA negatives. This finding held after stratifying for age and gender.
A restrospective study in the Philippines found a significant association between vitamin D status and severe Covid-19 infections.
An Indonesian study of 780 people found that after controlling for age, gender, and co-morbidity, vitamin D deficiency was associated with a twelve times higher risk of death.
Hospitalized male Covid-19 patients were found to have lower vitamin D levels than controls
A study out of Northwestern University found that the vitamin D status of a country's elderly population was associated with the number of severe cases of Covid-19 in that country.
A retrospective cohort study found that Chicago patients who were likely vitamin D deficient were more likely to test positive for Covid-19.
A observational study from Belgium found that vitamin D deficiency is correlated with the risk for hospitalization for COVID-19 pneumonia and predisposes to more advanced radiological disease stages.
A small cohort trial found that just 16% of patients who received vitamin D, magneseum, and vitamin B12 required oxygen compared with 61.5% of the previous cohort who did not receive DBM supplementation.

Comprehensive debate by the Irish Medical Journal:

Covid-19, Cocooning, and Vitamin D Intake Requirements Link
Authors of Article ‘Optimisation of Vitamin D Status for Enhanced Immuno-Protection against Covid-19’ by McCartney et al. comment on response report‘Covid-19, Cocooning and Vitamin DRequirements’ by McKenna et al Link
Vitamin D and Inflammation: Potential Implications for Severity of Covid Link
Authors of Article ‘Optimisation of Vitamin D Status for Enhanced Immuno-Protection against Covid-19’by McCartney et al. comment onresponse letter ‘Vitamin D and Covid-19: A Note of Caution’ by Rabbitt et al.Link
Vitamin D Deficiency and ARDS after SARS-CoV-2 Infection Link

This review of potential vitamin D mechanisms of action provides some useful history on the origin of vitamin D dosing recommendations. They also review their clinical guidelines after treating several thousand patients with 5000-10000 IU/day, including recognizing and resolving hypercalcemia or other adverse events.
Slovenia has begun administering vitamin D to residents of nursing homes and health care workers.
A review in Nature suggests that all patients should be monitored and potentially treated for vitamin D deficiency.
A bmj report which is generally critical of the connection between Covid-19 and vitamin D nevertheless concludes that supplementation according to government health guidelines (no more than 4,000 IU/day) is sensible for most people.
Evidence supports a vitamin-D causal model more than an acausal one.

Vitamin D deficiency is very common in places that have been hit hard by the disease.
Vitamin D reduces the risk of thrombosis which is one of the puzzling symptoms of Covid-19
Kawasaki disease was weakly associated with vitamin D supplementation at birth and KD patients had lower levels of vitamin D. There is an association with Covid-19 and a syndrome similar to Kawasaki disease in children.

Black and Asian populations are at a high risk of Covid morality. Some studies have found a relationship between skin pigmentation an vitamin D levels, but others have found that there is no difference in bio-available vitamin D because of differences in genetic polymorphisms related to vitamin D–binding protein. Interestingly, vitamin D binding protein is itself associated with infectious lung diseases.
There are lots of other factors which vary by race and which make Covid mortality more likely. These do not fully explain the race gap.
BAIPO, which represents UK doctors of Indian origin wrote to the NHS advocating for vitamin D testing and supplementation for front line staff. They are providing all members with vitamin D on request.
Blacks/African Americans are 5 Times More Likely to Develop COVID-19: Spatial Modeling of New York City ZIP Code-level Testing Results
Excess African-American mortality in the US was higher in southern states than northern states.
A dataset of 6 million US veterans found that black and hispanic had higher rates of Covid-19 which were not explained by medical conditions or where they live or receive care.

Evidence that Vitamin D Supplementation Could Reduce Risk of Influenza and COVID-19 Infections and Deaths
Nature - Perspective: Improving vitamin D status in the management of COVID-19
COVID-19 Perfect Storm (Part II): Role of Vitamins as Therapy or Preventive Strategy in Aged People

UVB radiation, which the skin uses to manufacture vitamin D is associated with lower death rates and case fatality rates.
Covid-19 deaths-per-million appears to vary by latitude.
Latitude was not associated with a proportional increase in cases during one week of March.

Vitamin D helps regulate the renin angiotensin system which includes the ACE2 receptor that Covid-19 binds to.
Vitamin D may bind to the non-structural protein nsp7 on the SARS-CoV-2 virus.
An article in Nature outlines a number of potential mechanisms.
The Essential Role of Vitamin D in the Biosynthesis of Endogenous Antimicrobial Peptides May Explain Why Deficiency Increases Mortality Risk in COVID-19 Infections
Vitamin D is essential to the formation of antimicrobial peptides such as cathelicidin and beta defensins.
Lungs as Target of COVID-19 Infection: Protective Common Molecular Mechanisms of Vitamin D and Melatonin as a New Potential Synergistic Treatment

Vitamin D reduces the risk of other acute respiratory infections. One interesting finding of this meta-analysis is that people who were very deficient (less than 25 nmol/L) saw the largest protective effect with an adjusted odd ratio of 0.58 and a 95% CI of 0.40 to 0.82. These are the same populations which seem to be at risk for severe Covid-19.
Vitamin D and Influenza — Prevention or Therapy?
Vitamin D and the anti-viral state
Vitamin D Supplementation to Prevent Acute Respiratory Tract Infections: Systematic Review and Meta-Analysis of Individual Participant Data

Vitamin D levels between 2006-2010 were associated with Covid-19 mortality, but not after controlling for every other variable in the data set. I don't know what to do with this study because there's a causal story in which low vitamin D in 2010 increases vitamin D in 2020 because that group is likely to supplement.