Vitamin D supplementation is recommended for people who have low levels of the vitamin, such as those being denied sunshine by remaining indoors. For more information see the plain English summary and scientific paper here:
Key results
"Overall, vitamin D supplements reduced the risk of having at least one ARI from 42% to 39%. We also showed that vitamin D had greater protective effects when it was given daily or weekly to people with the lowest vitamin D levels: the risk of having at least one ARI was reduced from 60% to 32% in these individuals. Vitamin D was not effective in protecting against ARIs when it was given in large, widely spaced doses. Taking vitamin D supplements was found to be safe."
A recent update on current thinking about Vitamin D's role in protecting us from the effects of a Covid-19 infection is given here:
Vitamin D: A Low-Hanging Fruit in COVID-19? Medscape, May 17, 2020
Other Dietary advice is available - it may be worth reading this interesting article on vitamins from MedicineNet:
Boosting Your Immunity From COVID-19? These Foods, Vitamins, Routines Work Best