The only supplement I take is Vitamin D. I have too much trouble breathing to mask even in crowds. While I was still on prednisone (between 10 and 40mg) I did catch colds easily. Without prednisone, I have only been sick with a respiratory virus once approximately 14 months ago. I caught that virus from my husband. It took several rounds of varyious antibiotics to recover. I have gradually learned to relax a bit. I hug friends and family. They know to keep their distance when sick. I eat at restaurants multiple times weekly--I always have and I don't think it ever resulted in catching a virus for me. I regularly attend social gatherings and meet with friends. Today I attended a funeral with about 200 other people. I am more cautious in cold/flu/covid season and watch for viral spikes in my community. At those times I do things like avoiding crowded movie theaters. If I were on an airline during cold/flu/covid season, I might consider a mask, especially if it wasn't difficult for me to breathe through. I am ever-alert to someone sneezing, coughing, or blowing a runny nose. I associate pharmacies with sick people so I use one that delivers or has a drive thru. I do my own grocery shopping at my neighborhood store, but I let Walmart and Amazon deliver most everything else. I've had pretty good luck staying healthy just by keeping strangers out of my personal space whenever possible and using good hygiene.
Worrying is only natural, but remember to think positive, and be patient while finding your groove.