> The previous comment, however, specifically mentioned long covid. Biomarkers for long covid are known.
Biomarkers for long covid are NOT "known". The Iwasaki paper discussed in this article is methodologically questionable (if not poor), and this much better, well-controlled cohort study from the NIH found the exact opposite -- no indication of significant differences across about 150 different clinical markers:
https://www.acpjournals.org/doi/10.7326/M21-4905
If I'm being maximally generous to Iwasaki's group, the best I can say is that they've advanced a hypothesis that needs to be verified by others, using better methods, with better controls. We're a long, long way from "known biomarkers".
> Both patient experience and biological research confirm that long covid isn't psychological.
There is no such research to "confirm" this claim, and "patient experience" is not evidence. It's an assertion. In fact, one good study out of France found that "self-reported COVID-19 infection was associated with most persistent physical symptoms, whereas laboratory-confirmed COVID-19 infection was associated only with anosmia.":
https://jamanetwork.com/journals/jamainternalmedicine/fullar...
About all you can say about "long covid" is that most of the existing research is of terrible quality, and the little that is well-controlled and high-quality doesn't confirm the most hyperbolic claims. If it is something other than psychosomatic illness, we don't know what it is, or how to diagnose it objectively.