Rather that attempt a comprehensive review of all essential nutrients (vitamins, minerals, amino acids, etc) as therapeutic agents let just look at the history of ascorbate - vitamin C - and scurvy. I think the case can be made that our handing of flu is similar.

Ascorbic acid, which we refer to in small doses as "vitamin C", occurs in all forms of life, all animals, plants, fungi, bacteria even viruses. One of the most primitive of molecules (it looks a lot like glucose) and is implicated in hundreds if not thousands of reactions in the body as a catalyst (something that makes a chemical reactions go faster or to happen at all). All our tissue is made from collagen which is made up or arrays of ascorbate molecules. "Krebs" or the "citric acid" cycle is the sole source of all energy in the body - a complex reaction of two dozen chemicals we can make, but 3 we cannot make and have to eat: Ascorbate, Niacin and Citrate. All energy production in the body is rate limited by these three molecules and that's all energy for muscle movement, all energy for autonomic and frontal cortex (thought) brain functions and all energy for healing tissue. That is both the raw material and energy to assemble it relies on sufficient ascorbate. There is no such thing a an average amount, each individual uses a certain amount every day but, say, you get a cut you'l need a bit more, if you have a cut on each arm you'll need twice as much and so on. A disease that attacks every cell in the body will use a phenomenal amount. Mankind's long history with scurvy - known to the ancient Egyptians and Hippocrates, we have known how to cue scurvy but via a failure of institutional memory and some bad science we lost the knowledge of how to treat it then found it again in a cycle many times over until Pauling's paper on the molecular bond enables the field of biochemistry which then let us understand the role of ascorbate in the body. That is it took us thousands of years to learn how to cure and prevent scurvy beyond any shadow of a doubt: there are no cases of scurvy where vitamin C doesn't work nor are there any side effects, ever and deaths from taking too much vitamin C in all of history are zero. You can buy it in nearly any store that sells food and it's used commonly in industry as a food resevatove. Keep all this in mind as an understanding of what a "safe and effective" treatment looks like.