Every day I start off with a nice glass of liquid extract of the fruit of the orange tree
So orange juice?  I'm a low-carb person so no juice for me, but I do take vitamin C
I also take Vitamin C even though Evidence does not support use in the general population for the prevention of the common cold
I take it because I don't want to get scurvy - LOL!  I don't get much eating keto (meats and green veggies)
Scurvy is nasty. When I start feeling the early symptoms I take more Vitamin C. 
I think a lot of good immune helping things are in combinations of Vitamin C and Zinc.
There is evidence that regular use of Vitamin C may shorten the length of colds
I think it's a good thing to take all the time - just in case!