Pity for those who take the shots instead of the summer sunshine and winter D-3 supplements. The shots vary from minimally effective to 70% or so each year according to the NIH. The Vitamin D is 100% and good for ALL seasonal viruses. A megadose of D-3 in my experience (x3 with other people) is effective in ending a case of flu no matter how severe in under 12 hours.
I have been taking 5000 IU of d3 for years...just recently ‘upgraded’ to the Vitamin d3 produced by Innate Formulas. Pricey and hard to find, but their formula incorporates k1 and k2, and is worth every penny.