Unfortunately, there’s not shortcuts I’m aware of. Once you feel it coming on, it’s too late. Emergen-C is a scam. I used to feel Zicam helped shorten a cold but when I dug deeper I realized if there was any material affect, it was likely placebo. (Which if either of these worked for you in the past and you believe they work, placebo is a valid healing path is some cases).
Rest is the best course. Your immune system needs sleep and nutrients to do its thing best. Minimize activity where you can. Exercise will make your symptoms worse. So chill and alleviate symptoms as you can.
