Not sure if anyone has linked this - a paper saying many severe coronavirus patients are vitamin-D deficient.
https://www.medrxiv.org/content/10.1101/2020.04.24.20075838v1
Obviously it's one paper out of many and caution should be taken on interpreting the results, but interesting nonetheless. It's suggesting for example that it might be one of the contributory factors to the large number of severe COVID-19 cases in the African American population (accordingly to the paper 80-90% of African Americans are vitamin-D deficient).
Personal experience - I've been taking vitamin-D supplements since the start of the year. My feeling is that for me, it substantially helps mood during the dark winter months. I don't know if it really affects my cold/flu symptoms (although interestingly I get far fewer colds and allergies since moving to Canada around 11 years ago).
Originally Posted by: pfw