Post-Exertional Malaise in Post-COVID-19 Syndrome: A Shift in the Frequency Across Pandemic Phases

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.