“Post-exertional malaise (PEM)
“Post-exertional malaise (PEM), which is the cardinal feature of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), is also reported in a proportion of patients with post-COVID-19 syndrome (PCS). Our objective was to identify determinants that may be linked to the emergence of PEM in PCS patients.”
*first sentence is very important 
“Patients who contracted COVID-19 during the pre-Omicron period reported PEM more frequently than those infected in later waves. This observation could reflect differences in viral characteristics following the emergence of the Omicron variant; however, alternative explanations—such as increasing vaccination coverage, accumulating post-infectious immunity, or other unmeasured factors—cannot be ruled out. Based on the observed link between PEM and symptom severity, PCS patients should be systematically assessed for the presence of PEM.”
