As a clinical pharmacologist who spends a lot of time evaluating medical liturature for medical guidance in a hospital, I've been reading some pretty interesting, higher quality studies on Vitamin D relationships and COVID. There's long been some evidence regarding the correlation of low Vitamin D and respiratory disease, but there seems to be mounting, compelling evidence that those with a sup-optimal/insufficient and deficient Vitamin D levels are at higher risk of COVID-19 and worse severity.
To be clear, that does not mean if your Vitamin D levels are fine, taking more Vitamin D will give you extra protection. But somewhere around 40% of Americans are Vitamin D deficient. This is especially true if you're of Northern European heritage, have darker skin, old age, obesity, low sunlight exposure (especially if in regions closer to the poles -or- if your lifestyle keeps you indoors), etc.
Vitamin D levels are not standardized, nor is there an optimal dose known for immune health. Levels are largely based on their relationship to bone health. The Endocrine Society however considers VitD insufficiency as having a serum 25(OH)D level of 21-29 ng/mL and deficiency as < 20 ng/mL.
The course of action on VitD deficient patients is usually to Rx 50,000 IU (1250mcg) of Vitamin D2 (Ergocalciferol - RX only) or D3 Cholecalciferol (OTC) once weekly for 6 weeks, followed by 1,500-2,000 units/day (37.5-50 mcg) maintenance. This may be increased however for specific patient populations (obese, malabsorption issues, etc).
This is of course is something not to be done unless you have a known deficiency (lab confirmed) and under the guidance of a doctor. There are populations where these typical VitD supplements are not ideal (i.e. kidney or hepatic disease). Overdosing on VitD can also cause problems as it increases Vitamin C absorption, which can cause a host of side effects that range from mild to possibly severe.
This may be an increasingly moot point considering vaccinations are here, but something to consider with your provider if you have ever been told you had a deficiency in the past or have had family members with this problem.