Vitamin D modulates the immune system and thus prevents a cytokine storm!! No worries about taking too much unless you have kidney stones like me...
As for elderberries, no one has proved or produced an example of where it caused a cytokine storm. I gave it in huge amounts to my 3 kids who have recovered from what must have been the swine flu and none had trouble with it--in fact, they recovered in 3 days, never got bad coughs, etc. I credit the elderberry for it!!