Interesting. Someone should (or maybe have?) run a cluster analysis on the symptoms to define more specific subgroups. But I suppose getting access to the required health data at that scale is nontrivial?

It’s not that hard to get a long list of symptoms for long covid. Just watch this thread as it grows, and you’ll easily find dozens. Things like this end up being a lint trap for people who just feel bad for whatever reason (which is all of us, at various points in our lives!) Nobody likes to be told that their symptoms are idiopathic.

Massaging this kind of data (clustering, etc.) is much lower value than finding fixed criteria that define a consistent group of patients who have objectively defined symptoms that cannot be more readily explained by another diagnosis. This is a pre-requisite for any further study. It can be done, but it’s hard, and it tends to lead to criticisms because you end up excluding a large number of people who fervently believe they have the illness, but don’t fit the objective standards.

Just for example: it’s not enough to claim that you have “brain fog”. A more valid endpoint might instead attempt to classify people based on standardized tests of thinking. Even that has problems, of course, but if you can just claim that you are fatigued and unable to think clearly, there’s a huge problem of confounding (i.e. maybe your symptoms are caused by something else), let alone the unverified nature of the original claim.