You are presuming that I believe that any amount of supplemental vitamin D should prevent viral infections.
Not so. Won't cite it here (too lengthy) but unless you attain 40 to 50 ng/ml of serum vitamin D, the immune system is compromised.
Generally (but with wide variation), each 1000 IUs of supplemented vitamin D yields about 10 ng/ml of serum vitamin D.
Look at the list of vitamins on your multi-vitamin bottle? How many actual IUs do you take each day?
Most multivitamins have no more the 2000 IUs --- enough to stave off rickets, per the standard vitamin D dogma. But insufficient for the immune system. Immune efficacy is dose-dependent, not merely dependent on the presence of vitamin D. Too little is, too little.
Have you had your serum vitamin D level checked? That's what's actually controlling. What concentrations of vitamin D are circulating in your blood stream?