My ME/CFS was triggered by an influenza vaccine. I have no doubt about this, as I experienced a mild shock within the 20 minute wait period after the vaccine, and then 2 days later I had acute Systemic Inflammatory Response Syndrome (SIRS/Sepsis). Experiencing acute SIRS symptoms was truly frightening. I also know a colleague whose ME/CFS was greatly worsened by an influenza vaccine. Vaccines are known to trigger ME/CFS, but by no means as common as viral onset of the disease.
It is described that the 1934 Los Angeles County Hospital outbreak of ME/CFS involving 198 staff, at the time called atypical poliomyelitis, was a result of vaccination with serum of recovered polio patients. Two of the affected doctors sued the county and were awarded $2m each, worth about $40m today! The outcome of this lawsuit has led some to postulate that subsequent ME/CFS outbreaks which were labelled as hysteria were deliberate smear campaigns to avoid massive compensation claims for disability.
Most healthcare providers are dismissive of vaccine risk and illness claims because they say it isn't a live virus and cannot cause disease. However a vaccine is designed to trigger an immune response, it wouldn't work if it didn't, and for some unlucky few like me this can lead to an exaggerated immune response. Near the time of the vaccine I had been suffering quite severe symptoms of gastritis and dyspepsia, I suspect enterovirus or some prior infection was involved, and the vaccine must have just tipped me over the edge.
I'm not anti-vaccination. You only need go back 100 years to know how comparatively dangerous the world was before vaccinations and how many children died before reaching teenage years. However I think it's also important to know that while the risk is low, in a minority of cases especially if a concurrent illness is occurring, that vaccinations can lead to undesirable outcomes. With respect to Covid vaccination, I admit I am anxious to get it, but on balance it seems it is considerably less risky for me than it would be getting a Covid infection!