A friend was commenting to me yesterday that she never used to get sick, but after covid she'll come down with every little thing. Must be the years of isolation, immune system just isn't back up to snuff.

I don't think that's it. My immune system is just fine. She is vaxxed to the max. 

 Someone I know is sick right now, probably just a cold, but she complains that a cold used to go away in a few days but now it lingers on. She's also been "vaxxed" and boosted, so perhaps that's a factor.