Swampy had cat flu twice as a kitten and has been left with cold symptoms all his life, mostly eyes (when younger) but now that he has CRF, the extra urea toxins cause wheezing, coughing and vomiting up phlegm. The vet suggested L Lysine but I can't say I really noticed any difference and it's another (big) tablet so I stopped. I think I tried echinacea but found it useless - both on me and him. If the eyes become a huge problem, there's an expensive eyedrop that boosts the immune system with Interferon. The veterinary one is Virbagen - Swampy had the human alpha interferon in drop form for years and it worked wonderfully on his cold.....but I do wonder if it didn't help to predispose him to liver disease eventually.
My ex vet in SA (brilliant vet) said to avoid the interferon injections as they are mostly a waste of time and money and can be harmful.
Mostly now that there's no treatment, I just control the eye infections with fucithalmic and try to keep the place warm and ventilated. Also steam inhalations (I put a bowl of steaming water with a few drops of olbas oil as near his nose as he will allow!) and keeping him for 5 - 10 mins in a steamy shower area is supposed to help. You can put them in a carrier covered with a towel with the olbas oil inhalation outside the carrier too (they don't much enjoy it!). Alternatively you can do what I dream of doing - move to a warmer, sunnier climate! Really helps with a cat cold.