Best flu preventive I know is the alkalising drink at the Ask Moreless forum. Kelps seems to work better for me than the green food you mention, cheaper where I get it, too!!!
Eating raw veg will ONLY help if one's digestion can handle raw food. Many with compromised immune systems also have problems with uncooked items, juiced or not.
Another issue if the general extremely poor quality of the food supply. If the veg one uses for this are of the usual abysmal quantity, they will not help at all, but make one worse.
When eating poor foods, I very much doubt that homeopathics or anything else will help one much. Remember, that when these worked, agriculture and food quality were very diff than now!!!
To deal with the quality issue, go to the Ask Moreless forum and search re: quality, refractometer, ask questions.
I KNOW I am strong enough to resist just about anything that comes along after a bit over two years using the ideas there!! Main thing is finding and/or growing the very best quality fresh food one can!!!!