Anecdotally, I felt like I had the cold coming on earlier in the season, I did a 30 hour fast and had limited to no cold symptoms after.
My working theory is that it gave my body the chance to focus on the virus and not digestion.
I should note that prior I was only doing 16/8 IF
However that is just my personal experience and I have no data or credible sources to back it up.
my gut reaction would be to either not fast, or have a “half” fast where you consume plenty of chicken soup and other foods good for when you have the flu, but no other foods (ok yeah it’s not really a fast, but it’s at least healthier eating?). you could also do a dirty fast with chicken broth, as that’s known to be good for when you have the flu/cold/etc.