Professors Tracy Hussell, Sheena Cruickshank, and John Grainger. We are experts in immunology - working on COVID-19 - and work at The University of Manchester answer questions on REDDIT. If you read down through the Q&A you'll find vitamin D3 recommendations that are interesting. There is also an explanation as to why there is not and probably will not be a common cold vaccine. All in all an interesting discussion. https://old.reddit.com/r/IAmA/comments/k22ka1/we_are_professors_tracy_hussell_sheena/ | 
I guess if you live in the USA take vitamin D3
