just a personal note, I had many many respiratory tract problems - tonsillitis, flu, cold, at least 3 times a year. I always thought it was something natural, given the seasonality of these diseases, especially in winter. but after a routine blood test, my doctor prescribed vitamin D because mine was way below recommended. since 2017, when i started taking vitamin D supplementation, only a mild flu is what I had in 3 years, so to me that article is no surprise, of course in a personal experience level.