I'll put it another way: long COVID has been studied quite a lot over the past 5 years, and I'm not aware of anyone being able to distinguish it from ME/CFS (except by definition). People appear to have stopped trying to draw a distinction, by and large, in favour of trying to identify better category boundaries to use instead. See https://doi.org/10.3390/biomedicines13112797 for a Nov 2025 literature review, which basically says "except for long COVID being caused by SARS-CoV-2, it's very difficult to tell ME/CFS-like long COVID and ME/CFS apart" in excruciating detail. Some things have been found to occur in long COVID but not found to occur in ME/CFS, and vice versa, but afaik there's nothing found to occur in ME/CFS-like long COVID that's been found to not occur in ME/CFS.

(Technically, long COVID is a broader diagnosis, encompassing some long-term conditions caused by a COVID-19 infection that are distinct from ME/CFS, but I consider that a "by definition" distinction rather than anything real. This is what you'd expect if ME/CFS had multiple causes, and COVID-19 infection could cause multiple chronic conditions, and most long COVID is actually ME/CFS.)

The ME Association came to this conclusion a couple of years ago: https://meassociation.org.uk/2023/05/updated-booklet-long-co...

> The ME Association (MEA) takes the view that Long Covid and ME/CFS are both examples of a serious and debilitating condition that can follow any type of viral infection.