I got Covid (Delta) in November despite being vaccinated. I guess you can call it a "mild case" - I felt sick for 1-2 days, then weaker than normal for ~ ten days. But I did have complete loss of smell - interestingly enough I could taste food just fine, but I could put my nose (which wasn't clogged up or anything) into a jar of ground coffee and wouldn't be able to smell a thing. No cognitive impairment though. And, if you read the article, it doesn't say anything about "brain damage" either, just "Olfactory Tissue Degeneration and Microvasculopathy" (which could be roughly translated as "nose damage", I guess) - no idea why it was submitted with this title.