Too late.  Have been on Vit D3 / K2 supplementation for years.  Haven't had a cold or flu for.... Years.   My Serum level is always on high side of the Bell Curve 

   Why test ?-  Because individual D - level is not tied to dose,  we're all very different in how we absorb or generate it.
  (K2  "re-directs"  calcium into bones, not arteries)
"Deficient"  is regarded as   <30nmol/L,    50  and you're looking Good,  70 and beyond is Jolly Useful  and to be desired.   Over 120  and you'll rarely get any 'flu.
The Yanks use a funnier measurement,  ng/mL,  in which you want at least 30  to feel safe.  70 is great !
Dose advice ranges from 2,000 IU  to 10,000IU.     Dr  Tony FAUCI  admits to taking 6,000IU.  -  He does not have covid (That we know of...)  but then again,  he appears to be in excellent health - for any age above 50.  I think he's 80 - something.
IF you take a  Statin,  then pills/Cod Liver Oil supplements are  useful,  as low cholesterol = lower sun-generated Vit D. 
 
How useful ? -  higher initial Vit D  correlates with milder symptoms / no infection  than 'Deficient'  numbers.  -  I'll take every advantage I can get, at my vintage !  Some treatment protocols use high doses of D,   10,000 or more.
Gosh, the things we find on a 4WD forum ! 
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