I take quercetin, zinc, vitamin C, and magnesium every evening before eating. Haven’t had a cold so far since I started this regimen months ago, but I have bad luck, so...
I’ve been taking a lot of vitamins since covid as well. Not only have I not had a cold, but I have not had a cold sore either. Knock on wood.
Don’t forget Vit D and Vit K.
Petey