There is a bit of research that says that good levels of vitamin D could speed up recovery of patients with COVID- 19. Not a prophylactic or a cure, but helpful.Most of this story is probably not worth reading, but it does address the question of vitamin D which @Norsemaid has raised earlier.
How risky is it to go shopping now? Your coronavirus questions answered
Long story short, there's no evidence that vitamin D will help prevent or cure COVID-19. There were some poorly done studies which suggested that it did, but they fell apart when looked at more closely.
Low vitamin D levels don't make you susceptable to COVID-19. Rather, older people or people who are otherwise in poor health are more likely to have low vitamin D levels for other reasons. These tend to be the same people who get more serious cases of COVID-19 anyway.
I won't tell people to not take a balanced and reasonable amount of vitamin supplements if their diet and health warrants it, but taking extra vitamin D is not going to be of any help with respect to COVID-19.
It will not prevent infection, but if you catch it your survival chances will improve.
"Short term, high-dose vitamin D supplementation for COVID-19 disease: a randomised, placebo-controlled, study (SHADE study) | Postgraduate Medical Journal
It is plausible that vitamin D supplementation may decrease the likelihood of SARS CoV-2 infection or cause an early viral clearance. It is noticed that vitamin D levels>30 ng/ml are associated with a significant decrease in the SARS-CoV-2 infection severity and mortality."
Therapeutic, high-dose cholecalciferol supplementation led to SARS-CoV-2 RNA negative in additional 41.7% participants (p<0.001) and was useful for viral SARS-CoV-2 RNA clearance.