I also have both, although I would rather put it as POIS with a CFS aspect. I shared my covid and vaccine experiences in other threads.
What got me interested is that you mentioned that
metformin deteriorated your symptoms. I checked what it is and it turns out to be another anti-diabetic drug with PPARG agonistic activity.
The anti-inflammatory effects of metformin inhibit the inflammatory response through downregulation of TLR4 expression and upregulation of PPARG activity.https://www.europeanreview.org/wp/wp-content/uploads/4988-4995.pdfThis is interesting as I suspect that many PPARG agonists (not all!) have a detrimental effect on my case. I also want to test pioglitazone for the same reason, although I am not sure if I am going to be able to get a prescription for it.
You also mentioned your relative with CFS who reacted well to metformin. Some other POISers also seem to react well to PPARG agonists, which may mean that PPARG is really a dividing factor in both POIS and CFS subtypes.
It is true that metformin also downregulates TLR4 expression and this is supposed to be beneficial in severe covid infection. Glycyrrhetinic acid is also a TLR4 antagonist and licorice actually helped my POIS case, although it may not have been due to this particular property.
It is also proposed by researchers that COVID-19 downregulates PPARG activity, but this has yet to be proven.
So I don't exactly know why she had a severe reaction to the vaccine, but it seems quite reasonable to think that PPARG and TLR4 were involved in it.