This seems to be the issue - we're trying to study COVID, which affected literally the whole population of earth at this point, and we have no control group of people who have not been affected.

People have gotten older, have gone through mass societal change, and any other number of things in between.

Also in every study we need a control group. How do you know for sure that the control group didn't get COVID. For all I know there is no way to know for sure you haven't been infected or sick of Covid in those 4 years, it is not like testing has been very reliable.

I know people who have been in long term covid studies since maybe April 2020, in what was the Seattle Flu study: self testing at least weekly back then and sending in samples regularly. The first people recruited were all the medical researchers at that institution and nearby who worked on things with no connection to covid, but relevant enough lab knowledge that they were trusted to understand how to self test.

> How do you know for sure that the control group didn't get COVID.

I dunno if that is relevant - it's the symptoms (brain inflammation[1]) that is hypothesised to correspond with lower cognitive function.

Even if you did have COVID, but didn't get brain inflammation[1], you can still serve as a control.

[1] I say "brain inflammation"; the exact mechanism is explained by a comment upthread.

I know for damn sure I didn't get COVID yet. I don't go outside I don't work I haven't since I really graduated from high school years and years and years ago because I'm disabled and I got three vaccines so far but the rare time I leave the house for groceries I'm always fully masked in a 3M n95. I've never came down with symptoms and I live alone, I'm pretty ideal unless I got asymptomatic at some point.

About a third of infections were completely asymptomatic, so that's not unlikely at all.

Yes, but you are also very different from the average person. So not real useful as a control group for typical people.

And some people might be faking it.

I always thought "getting old" was in part an accumulation of damage from infections and slow wear on the body.