Study suggests the "90% who are fine" may not all be fine.

> Our general approach in this study was therefore as follows: (i) utilise brain imaging data from 782 participants who have visited the UKB imaging centres for two scanning sessions, on average 3 years apart, with 394 of these having contracted COVID-19 in between their two scans;

Have they established with certainty that in the control group there were no-one with COVID19 infection in the first place?

Having a few false negatives in the control group shouldn't really matter since that would only make the groups more similar and hide any differences that exist rather than hallucinating ones that don't.