The 4 vitamins:
Vitamin C, Vitamin D, Zinc, and Quercetin
D3 with K2 is the most bioavailable form of Vitamin d meaning your body will absorb it most easily when these two vitamins are combined – eating healthy fats at the same time you take your vitamin D is also important for nutrient absorption since these are fat soluble vitamins.
Buffered vitamin C is more bioavailable or readily absorbed than ascorbic acid, which is the form of vitamin c in many supplements.
Don’t forget you can get vitamin c in your diet from foods like kale, bell peppers, and strawberries
Zinc has been shown to be a major factor in patients who are without their smell and taste for an extended period of time regaining that function and quercetin increases the absorption rate of zinc.
Anecdotally, change has also been seen in “long haul” COVID patients that supplement with glutathione and N-acetyl Cysteine